If you want portable, the gameboy is the way to go-the playstation means you need to bring along a TV and it needs to be plugged in and everything. That being said, the game boy is probably not 125 dollars canadian (About 100 US dollars, no?) You're talking about the game boy advance, which is newer and better than the gameboy original, but can play all original game boy games as well as game boy advance games. You can probably get an original game boy for much cheaper.
However, the PSP-or Play Station Portable is very small and easy to carry around-still needs a TV, but it's not much of a hassle. The PSP is not the newest playstation out there, there's a Play Station 2 which can again play newer and better games as well as all old playstation games. It also has the bonus feature of being a DVD and music CD player.
In my opinion, the playstation or playstation 2 will get you more value for your money-the games generally take longer to complete than most game boy games and also has much more multiplayer ({so your kids can play together), but it really is up to you-it depends on if the portability, etc of the gameboy is worth it.
One thing you might not be aware of, though, is that you might not need to buy a system after all. Movie rental chains like Blockbuster and Hollywood Video (Or a canadian equivalent) will let you rent game systems for much cheaper than it costs to buy them-so if you're going up to the cabin for a week, you can just rent a system for a week and save money. If the kids are going to be playing it at home, or if you're traveling extensively, you might save money by buying a system though.
Last notes-You might get a better reply by posting on a dedicated video gaming board, or by asking your local video game retailer for advice.