I have A LOT of experience with Hitec servos. They make some of the biggest bang for your buck servos. I use them a lot for my robotic applications. My favorite is the HSR-5990TG which has gobs of torque (the most powerful one out there in its class) and is far cheaper then any other brand such as KO, Futaba (they are probably the most expensive for what you get), and Kondo...
Back to the subject. The 645MGs are very reliable servos. If you're not going for response time (or transit time per 60-degree travel as it's commonly rated for servos), then the 645MGs does well.
A little background on the Hobbicos and Tower Hobbies servos. Majority of the Hobbico servos are rebadge of the Hitec servos. Just look at their CS-70 servo, it is a rebadged Hitec 645MG.
Their CS-170 digital servo is a rebadged version of Hitec's 5955TG servos (not to be confused with the 5990
The only difference is that CS-170 cannot be programmed by Hitec's digital servo programmer (not that you would need it for rc car application).
Tower Hobbies servos are often a rebadge of Hobbico's at a higher price. I use their TS-70 servo, which is a rebadged Hobbico CS-70, which is a rebadged Hitec 645MG servo... Once in a blue moon, the Tower Hobbies servos will be lower in price then Hobbicos. Usually, Hobbicos are much more cheaper then Hitecs. If the prices are the same between Hobbicos and Hitec, I'd go for Hitecs but there's no other reason to if the Hobbicos are generally cheaper.
Hobbico only lacks some of Hitec's latest greatest line, which are the robotics servos (same size as the standard servos we use in our 1/10th car), which are the HSR-5980SG (steel gear) and the HSR-5990TG...
Some of Hitec's earlier high-torque servos had thermal issues where the FETs inside would just blow out under extreme use (such as in Robotics), but if used for steering (say in an RC Car), it's quite a benign way to use it so I wouldn't worry. Hitec's 5995TG (not to be confused with the 5955
TG) would blow when going thermal, but their new 5990TG servo attempts to fix this by having a thermal shutdown inside. The 5980SG also has this feature, the difference between it and 5990TG is that it's steel gears instead of titanium, and the servo casing does not have an aluminum extrusion for heat sinking.
I highly recommend the 645MG if you don't care for a sub 0.2-second transit time... I use a rebadged, rebadged (Tower Hobbies TS-70) in my B44, and I also use 18 (yes, eighteen!) real Hitec 645MGs in my Hexapod Robot.
I hear many guys use AE servos in their ride. I've been meaning to check it out. Using servos in robotics (torque is the main feature you look for in robotics) versus RC Car (somewhat torque but more transit speed) is quite a different experience... Just remember to get minimum torque you need to move your front pairs in dirt (which takes quite a bit of torque, upwards of 133oz-in or more...)