I have two John Deere 350 three point hitch belt drive sickle mowers, one with a 9' cutterbar and one with a 7'. Other than the length of the bar the mowers are identical. The 9' is more or less permanently mounted to a Kubota 2400 HST, with 18pto hp. That is more than adequate power for the mower, even in heavy going. But the offset nature of a sickle mower means that virtually all the weight is on the right three point hitch lift arm. On the 9' the center of gravity is actually a bit outboard of the right lift pin, so the mower is pushing down on the right pin and lifting up on the left. The
B2400 handled the 9' on the TPH, but only after I replaced the OEM turnbuckle stabilizers with a more robust homemade system. Even then I felt that the
B2400 TPH was being abused, so I made a backhoe type subframe attached to the axle housings to carry the mower. This works fine and the 9' will probably stay on the
B2400 during the entire mowing season (March through September).
I also have a Kubota
L2900 (29 db hp) that easily handles the 7' mower. Although I did not put the 9' on it, I am confident that the
L2900 will handle the 9' as well.
The point of all this is that your tractor will have more than sufficient power for either a 7' or a 9' sickle mower. And since it is larger and heavier than my
L2900, it should not be bothered by the offset weight of the 9' mower (which is only a bit more than the 7' anyway). So if it were me, I would get a 9' rather than a 7' if both were available. Assuming an effective cut of 6" less than the bar length, and a tractor travelling 4mph, in one hour the 9' will mow 4 1/8 acres while the 7' mows 3 1/6 acres, or about 3/4 the amount mowed by the longer bar.
And, if you are not happy with the 9', you can shorten it to 7' by cutting 2' off the cutterbar and knife and driling a couple of holes in the new cutterbar end to remount the outer shoe.
A loooong time ago I used both a John Deere pittman drive mower and a New Holland belt drive mower. Pittman drive mowers have three disadvantages: 1. They vibrate a lot due to the nature of the pittman drive, but I have never heard of any operator being shaken to death and I can't tell that the mowing I did affected my brain (no snide comments, please). 2. As Ken Sweet said, the cutterbar doesn't like to be more than a few degrees above or below the tractor, so you can't drive along the level top of a pond dam and mow the bank going down to the water if it is very steep. 3. The pittman arm itself is usually part or all of the safety system for the drive train; therefore it is made of wood and rough cutting may break one or two per season. But they are cheap and easy to replace.
The belt drive mower 1. has much less vibration (how much less depends on the brand and drive system). 2. Will mow with the cutterbar at large angles from the tractor, both higher and lower. But none (to my knowledge) of the older belt drive mowers came with a system to raise or lower the cutterbar to those extreme angles. Therefore, the advantage of mowing at extreme angles will be available only by adding a hydraulic or other power lift system. Several members of this board have done that and their pictures will give you a good idea of what is required.
The belt drive mower has two disadvantages: 1. They are much more expensive than the pittman mowers. Ken Sweet can give you an idea of how much. 2. The drive mechanism that eliminates the pittman drive (called a wobble head) is much more complicated than the pittman drive, and some of the internal parts of the wobble head are proprietary to the manufacturer and, even if available, are very expensive. The only American made belt drive mower still in production is the New Holland, and its internals are probably the simplest of the belt drives, so it is probably the best choice. The JD 350's I have have not been made in at least 10 years, but every part is available and given JD's custom of long term parts support, I believe these machines will outlast me. I don't know about parts for the International, New Idea, Massey Ferguson, and Allis Chalmers belt drive mowers, but other members can probably advise you on this.
Good luck and keep us posted.