hard to argue with mcgrath's record, he got wickets everywhere at a great average, it's basically the most statistically solid bowling record in cricket. ever.
but I would akram in an all time XI because he was just unique in his ability.
notice how I haven't really answered the question.
A lot of people aren't answering the question. So I'm avoiding the fence completely...
I like McGrath better - you'll think I'm biased, but I have my reasons McGrath produced far more consistently and didn't pad his wickets tally with tail enders like Wasim did (35% of Wasim's wickets were #8-11, McGrath 25%. 40% of McGrath's wickets were the top 3, Wasim was 31%)
Wasim of course was more talented and could produce stunning delieveries and bowled some unplayable spells. But he just as often produced lacklustre spells where he'd bowl half a dozen no-balls, sprinkled with plenty of loose balls. He had very little patience and seemed to be wanting a wicket with every ball. Wasim's frustration seemed to come very quickly if he wasn't quite getting it right, and I remember often seeing him gesturing at his fielders about how they should be doing this and that, standing a bit finer/squarer after the balls already been bowled and I personally hate bowlers that do that (Stuart Broad...) And of course there are questions that were raised during his career over ball tampering and match fixing - Wasim may not be guilty, but the fact that questions were asked is never a good sign. For Wasim to play all those years with guys who admit to tampering (Imran, Shoaib and Waqar) and he'd never done it himself? I'd be stunned. In contrast, McGrath never had those questions asked at all.
So basically while everyone acknowledges Wasim was a difficult opponent because you never knew when that unplayable ball or that awesome reverse swing spell was coming, that's a lot different than picking someone for your team that you want to play alongside and I think I'd prefer McGrath on my team every day of the week.