The TR-707 and TR-808 are too different to really compare them. The 808 was the last and greatest hi end analog drum machine. That's why it gets used so much for both hiphop and electronic dance music because it is 100% synthesizer sounds which can sound very fat.

On the other hand the 707 was the first budget affordable digital drum machine from Roland. So it had pretty lo-resolution (8 bit) samples as was often the case at that early era of digital audio technology. But I liked the 707 a lot back then. And there were some nice factory rhythms built in.

They are very different animals. It totally is about what sound you want to make.If you want that electronic dance beat pumping then you absolutely can't go past the 808. But on the other hand if you are trying to sound like a real drummer ... forget about the 808, that is what a digital drum machine is for.