I think the suspended maybe cut a little better, maybe less likely to dig in and such. Someone that has owned both may be able to have a little more input. I own a BX with a 60 inch deck and its suspended.
Kubota b2650, bx2200, L3940 (gone), New Holland FWD TN85, RTV 900
With the suspended deck you can change the cutting height with the twist of a knob. On the 2650, you must change the 3 guide wheels separately; a real pita.You can lock the deck up and use the 3 pt for other things with the twist of a knob on the suspended deck. On the 2650, you have to get off the tractor and pin the deck up; a real pita. In my opinion, those are the biggest differences. I haven't noticed much, if any, difference in cut quality.
B2650HSTC, Grasshopper 725D front mower Kubota G1900
Prior to this past July, I had a BX 2200 with a 60 inch suspended deck, since then, the B2650 with a 72 inch ground contact deck. While neither can maneuver like a zero turn, I do prefer the 72 inch ground contact mmm.
Normally I cut at 3-3 1/2 inches, so the gauge wheels' height adjustment pins seldom need changing unless you're doing loader work and other 3 pt. tasks. Then pin up the deck (2 pins) and the 4 gauge wheels. Those tasks become part of the routine change over to another implement and take 5-10 minutes.
Aside from that, the ground contact gauge wheels provide extra, excellent flotation and swivel with turns, making them less likely to gouge ruts. The fixed gauge wheels of the suspended mmm are to prevent scalping but my experience was that they gouged turf creating ruts on soft or wet turf especially when turning corners.
The 72 inch extends out far enough on each side of the tractor's wheels which allows one to mow closer to trees etc. without scraping off lights, mirrors etc.
Also, that extra foot per swath over the 60 inch suspended deck adds up on a large lawn, significantly decreasing mowing time.
Add Gator blades and if you can fashion an outlet cover, mulching capabilities are possible.
Add the Cab version of the B2650 and you'll mow in a dust free, quieter, air-conditioned and UVB sun protected environment (some UVA still gets through so you still need to apply broad spectrum spf 50 or higher sunscreen sunscreen).
The drive over feature of the 72 inch deck makes install easier but the swiveling gauge wheels also allow an easier push in from the side and for fore/aft adjustment. The fixed gauge wheels of the suspended mmm require far more effort to push them even if you rotate them the 90 degrees allowed for mounting.
2009 Kubota RTV 900, 2009 Kubota B26 TLB & 2010 model LS P7010
Huskerfan made some good points. Changing deck height is not an issue with most folks as they set the height that they want to cut and it never varies during the season. He made enough pros that I would buy a contact deck over a suspended one IF I were in the market.
My turf is not a football field. It's uneven, rolling and hilly in areas. 1/4 mile long driveway with culverts along it that have steep angles with many trees.
I have used floating/ground contact decks for 45 years with great results and have no experience with suspended decks. It appears most Kubotas use suspended decks now. If spending upwards of 30 grand for a setup I want to make sure I understand the results I can expect.
I own both types. There are definitely pros and cons of each but they are sort of subtle. On the BX machines the decks are suspended and in my opinion do a slightly more erratic job than the ground riders. The reason is that the suspended deck rocks around up and down (left or right side lower than the other) when you hit rough spots or ruts and this gives a varying scalping or at least a varying cut height. The suspended decks do a beautiful job on very smooth lawns that do not cause the tractor to give you a "bumpy ride." Truth be known most of us do not have that golf green quality lawn smoothness. The ground riders provide a more consistent smoother looking cut in my opinion. They are not bouncing around in the air subject to rocking up and down left/right lower, etc. But with that said, the gound rider deck can also be sensitive to ruts and dips in a different manner than the suspended deck. You really have to try both to see the differences.
About adjusting cutting height: The suspended decks must adjusted with the knob under your seat ONLY when the deck is fully raised. Otherwise the knob is so hard to turn you will break it trying. You cannot just go running it up and down as you ride around. [The full weight of the suspended deck is on the knob-controlled stop for downward motion except when you have the deck fully raised and the hydraulics is holding the weight.] The ground rider deck has rigid fixed pin settings for the wheels for cutting height and that's the end of it. It depends how often you really need to change cutting height. Once I'm happy with it, I very rarely change cutting height anyway. I don't think you will find many commercial turf maintenance outfits running suspended decks. But obviously the suspended decks do work well for most Harry Homeowners.
My concern is cut quality on the 2. My experience is with 2 1985 Kubotas. An L2250 gear and then a B8200 hydro. Purchased the L and used it for half a summer and decided the constant shifting was annoying everyone in the family and the L turned out to be on the large side for the wife. So we bought the B and it's done most of the cutting for 30+ years. I don't want to make that mistake again. Both have floating point decks that needed replacing once in the 3 decades. They cut great.
The floating deck follows the contour of the land, the suspended follows the foot print of the four tires, which can make for vary different results.
Have been using a BX 54" suspended deck for mowing some pretty marginal ground since 2005 and I just do not see the side-to-side "rocking" JWR mentions. The consistency of cut is entirely adequate, and with over 6 acres to mow, we don't slow down. Just theorizing here... I always keep the gauge wheels set fairly close to the ground so any tendency to rock like that has minimal effect.
Those gauge wheels are a weak spot in the durability of any mowing deck, whether of suspended or ground contact design. If you regularly mow ground that has exposed roots, rocks and the like, repeated striking of those objects with the gauge wheels can eventually bend, break or otherwise damage them. Under such conditions, a suspended deck may be more survivable, since you can retract them out of harm's way.