I use both kinds of holsters and I feel that a regular holster with slots seems to ride slightly closer to the body, though that may be more perception than reality. To me it doesn't really matter too much, since you have to cover either one and I use loose fitting shirts for that. If you were trying to cover the holster with a tight fitting t-shirt then it might make some difference.
Also, if you're a small guy the regular slotted holster typically has less bulk and that could make a difference in concealment, but I've seen some pretty big ones too.
I'll say this, I use to have just slotted holsters and thought I didn't need snap on holsters at all, but after I got one I really liked it. I don't take my holster off and on throughout the day like some do, but it is easier to put on and take off even if I only do it once. I now have several with snaps.