Hopefully it's a question of 'when' and not 'if' (having just updated my Sony X10 on Vodafone, it's definitely worth the upgrade). I hoping, somewhat forlornly, that it might cure the random switching off problem that plagues the Ace.
Spoke to VM today about my phone, they've no idea when the update is due, in fact they had no idea if it would happen at all and offered to swap my Ace with the HTC Wildfire S, not sure it's suitable so I need to go look at one.
There isn't much difference between the two apart from the ace has a bigger screen with better display of colours, and a faster processor. It also has a bigger battery so should allow more use between charges. HTC's are notorious battery eaters. HTC's sense UI is considered far superior to Samsungs TouchWiz UI.
Anyway, you can have a look at a side by side comparison at the following link.