You will, more likely than not, pay for the $200 price difference over the course of the contract. If you're unlocked, you can move among GSM carriers (in this case AT&T and T-Mobile), and among pre-paid and post-paid plans according to your needs/desires/etc.
For example, the greatest cost savings, over the 2 years you have the phone, might be buying at full price and using it with one of the GoPhone pay-as-you-go plans. If the only people you call are AT&T customers, you do no messaging, and you do lots of data ... You could do unlimited talking and surfing for $50/mo. on a GoPhone SIM stuck into your E71. To get that on a contract plan, you'll have to take 450 Nation and PDA Personal == $70/mo.
A $20/mo difference is $240 in one year, $480 in two years. So, paying that extra $200 up front is going to pay for itself at the 10 month mark. If you're concerned about cost, clearly avoiding a contract is MUCH more in your interests than accepting the up-front discount that you'll have more than made up for in as little as 8 months.
If you throw in messaging, $70/mo for GoPhone, $90 for 450 Nation+PDA Personal Bundle. Same situation: $20 cheaper on a GoPhone plan. You come out WAY ahead.
What if you almost exclusively do surfing and no talking nor texting at all? $20/mo on GoPhone, $35 on contract. So, the price gap narrows a little, but the $200 price difference is still made up for in 14 months.
(and, in some cases, the carrier is giving you LESS than a $200 discount for taking the contract ... looks like AT&T mostly gives $150 discounts... you'll get your investment back in 8-10 months on a GoPhone plan)
And I haven't even thrown in the analysis of costs of "Pick Your Plan" vs contract plans.
On your "being without a phone while the non-carrier phone is under warranty service", I will say, if you buy the GoPhone packages on their web page, you can get a basic Nokia refurb for $10. Take the SIM and put it in your unlocked and factory fresh E71-2. If your E71-2 ever has a problem, you can use the cheap Nokia you got with your GoPhone SIM while the E71 is in the mail.
Obviously, the biggest concerns are:
1) What is your usage pattern? Especially if you're not using voice every day, or not using many minutes, prepaid plans can get VERY cheap compared to contract rates.
2) How much money do you have to spend up front? ... though, I bet anyone who is going to buy a smartphone can probably afford the extra $150-$200 to pay for it up front.
With AT&T, I can't think of a place where contract rates beat GoPhone rates.
With T-Mobile, it's not as dramatic nor clear cut, and depends a LOT more on what your usage pattern is (all night-time voice calls, no messaging nor data? Pay-Per-Day is $10 cheaper than 300 MyFavs or 600 Individual (if you're calling every night), so, yeah, you'll need closer to the full 2 years in order to make up the price difference, but you still come out ahead).
My only faults for the prepaid plans are:
AT&T doesn't include "Nights and Weekends" in their $1/day plan, just "Mobile to Mobile" and "basic voice access" (T-Mobile gives you all 4 for your $1/day: Nights, Weekends, M2M, basic voice access). I'd like to see AT&T include "Unlimited Nights and Weekends" in their $1/day plan. (of lesser importance, I wish their data and messaging add-ons for "pay as you go" plans were paid per day instead of paid per 30 days)
T-Mobile complete segregates voice and messaging+web on prepaid plans. If you want $1/day voice, you have to do pay-per-message for messaging, and no real web/data service (you can access THEIR web pages, to manage your account and download ringtones, and that's it). If you want $1/day for messaging/email/web, you have to have a sidekick, and all voice is pay-per-minute. I'd like to see T-Mobile combine these: any phone (not just sidekicks); $1/day for basic voice access, Nights, Weekends, M2M; $1/day for messaging, email, web.
If T-Mobile fixed theirs, and got a 3G E71 (getting back to the topic), I'd be their loyal customer forever. If they don't, and AT&T fixes theirs, I'd go with the E71-2 on their $1/day plan. If neither of them fix their issues ... I'm not sure which I'll do.
I'll probably get the E71-2, and one of those devices that lets you put 2 SIM cards in a single-SIM phone (there are versions of this that do it without having to cut the SIM cards), put T-Mobile Flexpay voice and messaging on one card and AT&T pay-as-you-go data on the other card. $75/mo for unlimited AT&T 3G data on an E71, unlimited messaging, unlimited nights, weekends, and 600 anytime minutes... all with no contract. (just for the record: unlimited nights and weekends is probably the most important factor for my voice usage patterns) To get that with AT&T, I'd pay $110/mo (though, I'd get a little more than 600 minutes).
The disadvantage to my plan is: I would have to do "voice and messaging" _OR_ "AT&T 3G data" at any given time (though, for t-mobile, email and instant messaging are included in "messaging", so that's not quite as limiting as it may sound). If I was ok with being limited to EDGE speed, I could use the E71-2 with T-Mobile only, and pay just $61/mo. Or, I could have it be "voice and messaging and EDGE-speed-web" _OR_ "At&t 3G data" ... for $81/mo. Either way, I get the one benefit of AT&T (3G data) for $29-$35/mo cheaper.
In any case, wasting my money on a contract just isn't in the cards.