Medela Pump-in-Style with the dual pumping. I believe other moms have mentioned that there are differences in the packaging -- sling bag, shoulder bag, backpack. The mechanics are the same. It's $279 new at Target/Babies R Us.
However, if this is the only time you're using it, you might rather rent one. Check your hospital, OB office, lactation consultant, or a local medical supply. Don't forget that you'll need to pump often while you're away to maintain your milk supply.
Of course, there is a chance the baby won't go back to nursing when you return (a weekend trip ended it for my 4 month old, but things weren't going well at that point anyway), but it's also highly likely that you can overcome any bumps in that road with very frequent nursing.
Given that chance, and if you'd want to keep him on breastmilk, you'd probably want to go ahead and buy one.
And even though the manufacturer says not to get a used one, I did, and used it for both of my kids, then a friend used it, and I sold it to yet another... All the parts are sterilizable and/or replaceable.