They sound pretty different. I would also recommend an Xotic Bass BB instead of the Fulltone, as it's tiny in comparison, holds more low end, and (apparently) sounds even better. If I had to choose between the VT/BB as my only pedal I would go for the BB as I love the sound of tubescreamer clones on bass, and not a huge fan of the drive tones you can get out of the VT. I use the VT as a pure amp sim rather than an overdrive; it makes my Markbass amp sound like a dirty old Ampeg and the high-cut really smooths out the sound of distortion pedals. Since Ashdowns can sound pretty dirty and old school anyway, and your cabs may have a natural 5k rolloff by themselves, I reckon you'd be better off with a decent overdrive.
You can also get a pretty flat sound out of the VT Bass (
see here) so don't worry about it overly colouring your sound. With the right settings it sounds pretty neutral apart from the 5k speaker sim rolloff.
If you can, get both and sell the one you don't like. VTs pop up pretty regularly on the classifieds here, although they tend to have a lifespan of only a few minutes before someone snaps em up!