I've just recently got the 45 litre xline trolley and I do really like it. At £600 though it's not cheap.
If you set it up with enough hose that you don't need to move the trolley much, then it is great in some jobs. I always found backpacks to be good but sometimes it would be frustrating running out of the 20 litres of water with a few windows to go and having to fill up again.
Only thing I'd say about the xline 45 trolley is that the handle is a bit low for us tall fellas. I'm 6"2 and have to stoop a bit to wheel it around. That's why I make sure I've got about 20 metres of hose on it so I can get lot done from one move of the trolley.
For me I wouldnt spend a lot of money on a trolley that could only take 25 litre barrels as it's just not enough water capacity to warrant the cost. The gardiner backpack has 22litre capacity and is only £100.