Both the Mtn. Fork and the Illinois are good for fishing. You just have to be aware that both will have tourons, party rafters on the Illinois and party kayakers on the Lower Mtn. Fork, when the weather gets warm enough.
The lower Mtn. Fork is about 3 miles from the Rereg dam to the highway. If you want longer you can always go from the highway to one of the outfitters takeouts. Lots of outfitters there.
The Illinois is probably the best Smallmouth river in Oklahoma. It has lots of launches and take-outs. So many it is often confusing. It also has some campsites at the launches. I don't know much about any outfitters there but there is almost always someone up there that can help if you set it up ahead of time. Join the group North East Oklahoma Kayak Anglers
on Facebook and ask them some questions. The Illinois is worth the extra driving time.
Differences in the rivers...the Mtn Fork is a rock bottom river. Usually these rock are very uneven(think ankle breakers)and slick. There are some minor rapids because of these rocks. The water is cold because it comes out of the bottom of Broken Bow Lake. It has year round trout in it. The Illinois is a gravel/sand bottom lake...for the most part and usually has a nice steady flow with little rapids. The water is warm above Tenkiller lake and cold below. The upper part is great smallie fishing. Never fished the lower but I know it has trout. More info Illinois info
If the Illinois was within 3 hours I would be on the upper every weekend.