Temple has one of the best IB programs in the country, and I would say for HIGH achievers, honors students, Temple High probably has the best academics in the area. Top notch teachers. However, outside the high achievers, Temple's state test performance has been low in recent years.
Temple dominates in fine arts - drama is a State/National powerhouse, and will be as long as Tolleson is there. Choir, band, and orchestra at Temple is also tops in the area.
Athletics - in football, frankly, both Temple and Belton aren't very good. Neither made the playoffs this year (and all you have to do to make the playoffs is be in the top half). Other sports - who cares, right? This is Texas...
Belton Middle Schools are top notch. New buildings and more construction/campuses on the way. iPads for all students at some of them.
Don't know much about Belton High, but I can tell you they have siphoned off 500 or so students to the down town campus. Also, I believe the current BMS will soon be part of BHS, with two new middle schools in the works.
The bottom line about the two districts is simple - Temple ISD, a proud and accomplished District, is landlocked and has no room to grow. I hope the best, but I expect Temple iSD in 20 years will comprise what TISD comprises now, except 20 years older. there is no room in TISD for new housing to be occupied by young, professional people with kids. Not good. Waco ISD comes to mind.
Belton ISD on the other hand has virtually unlimited land, much of which is in the Temple City Limits, ironically. Enough, really, for 2 or 3 5A high schools within 10-20 years. Unlimited growth, for the time being, anyway.
There are some small Districts that are OK. Rogers is good, and its boundaries extend all the way to east Temple. Salado is Salado. Academy is OK. Many of your new subdivisions in south Temple out 5th street are Academy ISD (see landlocked comments above).