AT&T does not allow tethering without paying for it, and they will cut you off if they detect it. I guess Sprint doesn't?
Sprint, it appears, charges $79.99 for 450 minutes and unlimited data (including the $10 smartphone premium). If you want tethering without jailbreak, that's an extra $29.99, and tethering is capped at 5GB - total $109.98.
AT&T would be $39.99 for 450 minutes, $25.00 for 2GB data, and $20.00 for tethering plus another 2GB data (4GB total phone/tethering combined) - total $84.99. This is "Basically" what I have (I actually have a 550 minute family plan but if I were on individual plan this is what I'd have).
So if you want tethering (without jailbreak), Sprint costs an extra $25 - and gives you 5GB tethered + unlimited on phone vs. AT&T's 4GB shared. If you jailbreak you can tether on the Sprint plan for $79.99 with unlimited data, or you can do AT&T 4GB shared for $84.99. You could even go up to 6GB on AT&T and still be cheaper ($10/GB overage).
Speed is a good 4x faster on AT&T. You're getting 0.68Mbps, which beats the 0.581Mbps average from
this article - which showed AT&T average at about 3.2Mbps.
Most of my tethering is email, facebook, maybe occasional YouTube on my laptop or iPad. Even with tethering 2-3 times a month I never come close to my 4GB cap, so personally I'm quite happy with the AT&T service - faster and cheaper than Sprint. Even if I were willing to jailbreak, I wouldn't give up the speed in order to save $5/month.
If your usage pattern puts you reliably above 6GB/month (or 4GB/mo and you're willing to jailbreak), then Sprint could be a better deal.