Tri-band refers to the radios installed in the device. Nothing to do with performance but rather the ability to use the device in different regions of the world with an international carrier, etc. I wouldn't worry about what bands the device has unless you travel out of your country often and use other carrier Sims in the phone (granted you have it unlocked)
Actually, nowadays, it depends on how the term "tri-band" is used in context. Before, tri-band always referred to an additional band to connect internationally. The term is not as cut and dry as before, due to carriers purchasing spectrums with different frequencies.
Sprint is going to start building out towers for what used to be Nextel and convert those to LTE (at 800mhz). Also, they just purchased the rest of Clearwire's spectrum, which is at 2500mhz.
Phones have to have built in radios in order to connect to those different frequencies. Given the fact that the purchase of the remaining clearwire spectrum has to be approved by the FCC, I doubt we'll see phones with the ability to connect to that spectrum for quite some time. The final details of the Softbank deal still have to be finalized (and that happened quite some time ago), so we could be looking at a couple years until Sprint starts building phones w/ the necessary hardware to connect to the 2500mhz frequency.
So, the "easiest" answer is that by the time the 800mhz and 2500mhz frequencies are ready for phones to connect, you won't have to worry about being upgrade eligible again, so you can buy a phone that will connect to those frequencies.
Another good example is those that have/had Wimax phones. When Sprint announced the switch to LTE, many worried that they'd no longer have 4g connectivity, but Sprint verified that they'd be supporting Wimax until 2014, which gave those that purchased Wimax phones in 2012 the ability to upgrade to an LTE phone, by the time they turned off the Wimax switch.
Hope this info helps.
