There are no two thoughts about Apple's quality and hence its dominance in the mobile market. But, given the competition and the quality products available from other vendors, it becomes absolutely sensible to compare before showing more loyality to Apple than the your own pocket!
If you are a proud owner of iPhone 4 and looking to buy 4S, i would say that giving a thorough look at the new offerings in 4S as compared to 4 would make sense. This is because, i don't see major changes as far as upgrade is concerned. If you are a first time buyer, you should offcourse go for it after giving a chance to similar offerring available in market.
Here are some of the key differences and commonalities that i think matter. [More]