Very logical of many choices. If you go with a tracked skid steer and rotary cutter you have a lot heavier duty set of equipment. Going to that expense I recommend that you get a HEAVY rotary cutter and make sure the skid steer hydraulic accessory pump is high-flow. They usually have multiple pumps. Those things will do major damage to underbrush and even 4" trees. I used to own a CID brand 5ft "Extreme" Brush cutter that was a real beast. See
X-treme Brush Cutter
I got it in 2011 for about $3500 but I'm sure they have gone up. Only reason I did not keep it was that my tractor (81hp MF 2660) could not produce enough hydraulic flow to make it robust. It clogged way too easy. Those are made well. Parker motors, 3 heavy blades on a 5/8" thick carrier, etc. With a skid steer and high flow you will be able to cut & clear big time. I think you should get a grapple for the FEL on your Deere which will be a huge help in clearing out trees, pieces of trees, debris etc. That will be very useful for the whole life of the Deere whether you still have the skid steer or not. I'll be very surprised if you do not also get a 5 or 6 ft bush hog for general purpose cutting of weeds, lighter brush, etc. with the Deere, especially if you have some open field areas.
I got a 66" Wildkat (same as Mid-state) brand grapple new 2 yrs ago for $1500 incl. tax. I find it plenty heavy, almost indestructible and so handy I keep it on my MF most of the time. For my 2 cents worth I would still get a skid-steer adapter for your Deere FEL -- even more so now that you will have a skid steer. That way you can trade implements back and forth between machines as needed plus have the huge wide choice of things that fit it. That includes buckets, grapples, you name it. Be sure to get compatible flat-face connectors for all the hydraulic lines !
I have no feel at all for implement availability, dealers, parts, support, etc. out there in Oregon. I am very familiar with mid-East coast, but not out there.
By the way, unless there is specific prohibition language in your Deere warrantee they cannot/would not deny your warrantee coverage due to something like a Lane Shark on your FEL. They can't legitimately. It is well within the load capacity specs of your FEL. If you can afford the skid steer route you will be better equipped to handle much tougher clearing tasks but I don't think the warrantee should have been a concern at all.
Best of luck with all this and please give us nosey people an update every once in a while as you wade through the next several months.