Sprint uses WiMAX as its "4G" services, which is very slow compared to its full potential. LTE will most likely be used by a majority of the market. By 2013 Verizon Wireless should have their 3G network upgraded to LTE. Sprint's WiMAX coverage area is restricted to... like 27 markets in only 15 states.
Sprint 4g has much greater capacity for data but is stuck with CDMA. I am an Independent consultant working on building out the broadband network right now. Currently I am working on att, Sprint/clearwire and Lightsquared. Good thing about Sprint / Clearwire is the amount of new equipment they are installing is very robust and has room for growth. Sadly it only works for broad band and will not improve voice.
ATT is replacing one antenna per sector, with a new "LTE" antenna, then adding two small radio units near the antenna as the LTE support. This spreads the current array of 4 antenna per sector (3 sectors total) to GSM, UMTS 850/1900 and LTE. I am not an RF engineer but it seems to me it is spread a bit thin considering each panel antenna can only handle about 250 calls at a time.
The build out is massive right now. I am seeing it first hand. In the last 90 days we have completed more than 45 att LTE sites for one small section of the Bay Area. Great news for what I do.
In the end, close to 2013 as you say, Verizon will incorporate LTE, and Sprint will be the odd man out, but still a very viable option. My money is on LTE. After all LTE means Long Term Evolution.