First of all, anyone still quoting 50:1 contention ratio is also being economical with the truth. ISPs shouldn't really be quoting contention ratios at all now as it's almost impossible for them to determine the ratio.
In a nutshell, contention ratio is a calculation of number of users versus available total bandwidth, to see how many you are effectively 'sharing' your 2Mb (or whatever) with. Because of the factors e.g. exchange traffic, ISP traffic etc, it's very difficult to tell true contention ratios nowadays in normal consumer broadband. Most of the 'mass-market' ISPs (i.e. pretty much all of them) are probably well exceeding 100:1 nowadays.
BT Wholesale abolished the publication of 50:1 and 20:1 contention ratios a few years ago - expecting ISPs to follow suit, shortly after the introduction of MaxDSL, funnily enough
So basically, I'd go on an ISP's reputation, rather than any quoted 'statistics' ... after all, 74.9% of statistics are made up on the spot, you know