I would try the Korker boots out with the interchangeable soles. Like you said, you can test out the felt...see how you like it, and test out the rubber soles...see how you like it. Like others have said TRY THE BOOTS ON with your waders on. Years back I was planning to buy a pair of Korkers, then went and tried them on at SeaRun with my waders on and did not like the fit one bit! The fit for wading boots is not as precise as a new pair of hiking boots of whatever, but you will still find that some brands feel/fit better than others.
Personally I love felt. I traverse slippery rocks a fair bit and find the grip of felt 2nd to none. However they do suck on trails esp if there is mud, wet leaves, etc. I have a 2nd pair of vibram soled boots that I've also loaded up with tractions studs. If I know I'm going to be doing a fair bit of bushwhacking I'll leave the felts at home. However for walking on rocks I they are very noisy/annoying and IMO do not grip as well as felt.
As others have said there are costs/benefits for each type of sole and the right one for you will depend of the type of fishing/terrain you do and personal preference.