Assuming that you'll be doing portraits both indoors and outside, you'll need the speedlight for sure. I use my Canon 580EX as a fill flash for shots taken outdoors (also puts a "catchlight" in the subject's eyes) and for indoor formals, I always use my Alien Bees mono studio lights with silver umbrellas.

The studio setup lets you determine before-hand how much shadow you want to create and where and then lets you concentrate on getting expression, ears, head position, etc. I've used the Bees for 4 years now and they appear to be bullet-proof (looking for wood to knock on). Also, they let you shoot at F8 or F11 which is the sweet spot for most lenses.

If cash is short (ain't it always) then go with the speedlight first and learn how to use it for fill and/or primary lighting then get your studio strobes later.

Jim