For short-term trading it's different, but from a long-term perspective we definitely need to be looking at the specifics of each project, what real-world solution it offers, what its development history and roadmap are like etc etc - these are the things that matter, so much more than whether it happens to be #1 altcoin in market cap at any given time.
I would say ETH and XRP both have huge potential, and can have a tremendous real-world impact. Both look cheap at the moment, but of course you want to know whether or not prices will fall further - which no-one can say for sure. For me the market does seem to have absolutely no support at the moment, so further losses can't be ruled out at all.