You don't want to shoot weddings with an f4 lens.
And that Canon 24-70 is a tough workhorse. If you are going to put it to work, just get a used mark 1 on on Ebay for $1200. Then save up for a 70-200 F2.8 IS - the first one is fine. Those are must-haves for weddings, from my experience.
I will say that Tamron 24-70 does sound nice with VC on it. VC will come in handy during weddings.
Bossier City, Louisiana
Shreveport Photographic Society:
Canon EOS 5D
Canon EOS 5D Mark II
Canon EF 35mm f/1.4L USM
Canon EF 85mm f/1.2L II USM
Canon EF 70-200mm f/2.8L IS USM