a jasper would probably run cooler, but there's not really much difference between a falcon or a jtasper if you're running a jtag/rgh. you can set temp thresholds. I have two falcon jtags and one jasper with a flashed drive. I only use the jtag, since going online doesn't interest me. there's a way to go online with a jtag/rgh, but you have to compile the app yourself, which means getting a hold of an xdk (xbox development kit). they contain copyright code, so you won't find them here. I've never tried to compile an xbox 360 app as I don't have the xdk. however, a temp member told me there's a 80GB torrent or so that has every one of them that's leaked. I'm hesitant to try it, because if it doesn't work and I'm banned...
there's also a stealth service, which uses a plugin through dashlaunch (think the free one above does too, but you run the server on your computer). it's a complete rip off, and I don't recommend it.
as for errors, I've only had one issue on the jtag I use. one time everything got corrupted. a friend of mine told me that supposedly fakeanim was corrupting xboxes including his own. he ran an rgh service. it never happened to me except for that one time, and I'm not even sure if fakeanim, which I use, was responsible. I still use it. I think it was just a random glitch. I managed to fix it, but it took a little time.
you're pretty limited if you just do a flashed drive, so I say go with an rgh. if you can't solder (don't recommend learning on the system), then I think there are several places that sell them for maybe $50 more than a normal used system. I haven't looked at the prices in a long time, but I'd personally go with a slim as they're more reliable. however, I only have experience with phats. the jtag I have plugged in has lasted about 10+ years thus far. I've replaced the thermal paste maybe twice, and it runs even cooler than my ps3 slim with temp thresholds. the recommend temp is 60 degrees C for the 360 while it's recommended 68 C or so for the ps3. the cell in the ps3 can endure heat better than the 360 chips, but both rsx and the gpu of the ps3 and 360 respectively can't. I wouldn't really worry about a failing system unless you got a xenon. that's first generation phats with no hdmi. they're crap, so I don't recommend them.