Before I answer, highly recommend the successor to v3.5 called Pathfinder. The makers of Dungeon Magazine (if you've ever read it, some of the best writers in fantasy gaming) are responsible for this edition, and it's compatible with 3.5, improving upon it in the direction Hasbro chose not to take. Unlike 3.5, new materials are constantly emerging, especially "adventure paths" that take groups from 1-20. You will be hard pressed to find better written material in the field right now. As an example of its difference, during the Beta of the rules, 50,000+ gamers contributed feedback to the company. It's an amazing community.
With that said, pros and cons.
3.5 pros: Free online rules, likely to find books cheaper, lots of online content still. You're already familiar with it.
3.5 cons: No more material produced for it. Several rules need fixed for game balance or ease.
Pathfinder pros: Free online rules, 1 game book (combines player and DM guide), terrific online community for support and resources. Game editors consistently offer input to gamers. Considered v3.75, so almost all of it will look familiar to player of 3.5. Able to use most 3.5 materials with minimal conversion. Constant stream of new products coming out. Top writers in the business contribute to Pathfinder. The main company Paizo makes or sells other fantastic game aids as well, like battle mats, critical hits cards, spell cards, etc.
Pathfinder cons: Books and adventure paths can get pricy, even if you order .pdf forms instead. Like 3.5, still a lot of rules for a new player to absorb.
4th Edition pros: Simplifies things (characters can fit info on a card), good for video-game generation of new players to grasp. Steady stream of products (more accessories than modules). Reduces # of skills to manageable level, encourages teamplay through use of skills. Website provides stream of free material to gamers as it did with 3.5.
4th Edition cons: Combats can be obscenely long, feels like playing an online game like World of Warcraft with abilities, characters are described in their role in combat. Less emphasis or material for use of special abilities outside a combat. Many released accessories are overpriced variations of earlier editions (nothing new). Absolutely no compatibility with version 3.5. No free online rules.