A couple suggestions:
1. Call NRS and ask them the pros/cons of the SB and non-SB, pros/cons of buying used vs. new, what to look for in a used raft, why size matters, etc: 877.677.4327
2. Call AIRE and ask them the same questions: 800-247-3432
Both companies have excellent customer service and can help answer your questions. They each sell and use both types of rafts for fishing and more.
3. Size of the raft is also important. For the primary use of fishing you may not want a 15' boat, you might want something smaller and more manageable, allowing you to catch smaller eddies and 'drift'.
4. Condition of the Used Rafts - look for patches, wear spots, fading, peeling patches, poorly applied patches (ie. more glue than patch), seams that are starting to separate, operation of valves. As for frame, how heavy is it, if it's steel does it show signs of serious rust (external or internal), what condition is it in, does it easily break down, or will you need a trailer or rack system for it, are any welds cracked or broken, etc.
5. Find a friend with experience that you trust to help you check out the used rafts/frames. There are used rafts out there for good prices that will give you years of enjoyment - and then there are other rafts out there that you can spend more time repairing than using.
6. Consider renting - either to try before you buy, or as an ongoing solution. If you're only going to use it on a few trips a year - it might be worth it to spend a few hundred a year renting instead of dropping $2K-$7K up front.