However, Sprint is lacking in both speed and coverage compared to their competitors (if you exclude WiMAX).
This is not correct. AT&T always advertise to have the fastest 3G Network but field tests from PC Mag confirmed that when it boils down to it, all 3Gs are more or less the same speed, give or take a 100+ kbps
If I end up getting the 4s, it is for Sprint. Why would you want to tether on your 3G phone? Get a 4G MiFi! With the $10 off special (if you get Mifi + phone plan together), you end up paying only $35 for the Mifi, only $5 more than tethering on your phone, no data cap on 4G and 3GB 3G limit