They are equal! Most of the members here say that separators are not relevant. But Search Engines are not that wise. They can't recognized keywords on very long domain names. Hyphens are important and the best keyword delimiter in your URL.
In your case, mensclothes.com would be better. If your domain name contained multiple keywords that could be mixed up, as is the case of Experts Exchange, then you'd want to go with the dash in the domain name. For an example of what I'm talking about, look at experts-exchange.com and expertsexchange.com - the first one is VERY easy to identify since you know what the keywords are (Experts Exchange). However, the second one can cause confusion - are the keywords "Experts Exchange" or "Expert Sex Change" - see what I mean?
I agree mensclothes would be better. They are equal and mensclothes is obviously easier to remember. As Dan said, for something like experts exchange the - may be needed
It wouldn't matter to search engines either you use a dash or not in URL, just keep it simple and easy to figure out for the convenience of both the search engines and users. On my personal view, you may omit the dash on your URL since it only contains two short words..
I would go with the non hyphen one first if you can get it, especially if it is a .com domain. Having those two things plus decent and relevant content should make it much easier for you to get on the first page for your keywords. I have seen websites that are crap but are able rank on page one for very competitive keywords because the owner was the first to snag the complete keyword without hyphens in the domain along with the .com. I seen this far too many times for it to be a mere coincidence.
The hyphen seems kind of spammy, for lack of a better word. How about mens-clothes.com vs. mensclothes.net? That's the dilemma people probably encounter most often.
The hyphen does act as a delimiter, however the benefits are not nearly as substantial as they used to be due to the search engine algo's getting better so unless they are very competitive keywords you are trying to rank high for you should just go with the non-hyphenated.