I wouldn't suggest anything Olympic. If you want a good 9mm sub gun, there's a couple decent options out there and an AR coming from Colt soon as well, so I hear. I had an MP5 and though it had it's role, it's not someone I'd choose for home defense when other options are available.

Everyone is different, but I take the defense of my family extremely serious especially after almost shooting those guys breaking in last year. So, I spend the money on quality (and training) and I prefer 5.56. Will defeat a vest but also not over penetrate and not be AS deadly after penetrating the interior/exterior walls as much as even a 9mm or 45.

If this is a range toy, go for it and enjoy. If it's a home defense weapon, don't make a choice based on magazine availability.