I have the 4S, and my brother has the 4. My iPhone is almost twice as fast when loading apps, multitasking, etc. My 4S's screen and battery is also better than his .4 If you could hold out maybe 4/5 more months, Apple will almost 100% be releasing an even newer model. Sprint is currently rolling out the Network Vision upgrade across the nation to its network, and neither of the current iPhone's support the 800mhz frequency that will be drastically increasing indoor and overall reception. I would by an andriod phone off of ebay right now to tide myself over, and wait for the new iPhone to come out and upgrade then, if I were you. Heres my motto with Apple, either upgrade within 3 months of an item being released, or wait until the next model is released. If you have pretty good coverage at home and you are happy with your current service, don't care about having the "latest and greatest", and don't mind having a somewhat smaller screen size, then by all means upgrade now.
Siri isn't the only draw for the 4S over the 4. Besides doubling the shared storage, your extra $100 nets you:
Siri stands out because anyone can find a use for it. But bear in mind that it's what we techies call a "beta release". Apple acquired Siri's devloper as the app spent but a short time on the open market. Once Apple brought the Siri team on board, it focused more on the hardware and how best the app would integrate with the hardware. The whole concept of the software backend recognizing and being able to parse natural speech is amazing, and yet it may have a long road to travel before iit's ready for common men to use.
Edited 6/3 by NAlden: for clarity regarding camera quality
I would go for Siri. We have! Using her while driving and asking her to set reminders and make calls all voice activated and she'll make a safer driving experience as well as not having to type, even if not driving, that's a real plus.
Voice texting is lovely! Definitely worth the $100 just to have the pleasure of saying, "do I need an umbrella today?" and "where's the closest coffee house?"
I get my upgrade tomorrow (yeah!). However I am stuck between these two Iphones. My friend has an Iphone with the Siri, and I tried it today and I did not really think it was worth an extra 100. I want to use my phone for apps mainly. I am upgrading from a Replenish, I am sorry I bought that phone. I think Sprint ought to recall it and let everyone who bought it get a different phone. But I digress. Will I notice a huge difference getting either one over the latter? Thanks!