I have my stand alone effects at home while I've got my Multi-FX (BOSS ME-50) at school (college, dorm-life, etc.,).
I only use the ME-50 so I don't have to take multiple pedals with me to school just to the sounds I want. I also don't want to risk losing them either...you know how "friends" can be right?
Speaking of friends; if I go to some friends house and jam with them during the week I don't have to go clear across town just to get my stuff. I have the guitar, the multi-effects, and a small amp.
Plus it has a headphone jack, so I can play late at night w/out waking the neighbors.
I like individual effects a lot more than Multi-effects mainly because I think the individual pedals have more character. The Multi-effects I've played all lack a certain "something"...maybe I'm playing the wrong ones or looking for the wrong things?
But, I suppose if you're a gigging guitarist that needs to produce a lot of different sounds on the fly, added to the fact that the audience doesn't usually care about your tone just as long as it's listenable (they don't know the difference between the GT-6 phaser and the Small Stone), then maybe a multi-effects unit if right for you.
I'd go with either the Boss GT-6 (but if menus isn't your thing use the ME-50) or the Digitech GNX4. Both are fine products.