There is nothing horribly wrong with one size up (and I know people who prefer it to be bigger) especially for stuff like trail riding, but personally I don't like when it's too big (I move around too much). If it's a very good price or free, then why not? Otherwise I'd just keep looking.
It's personal preference. I like a bigger seat, but my husband likes one that's "snug." I'd ride in it if you can, to see if you like it. If it's going to be shipped, then I'd only get it if it were cheap enough that you could re-sell it if you didn't like it.