I would definately go for the purefreedom 50 litre trolley.
We live near PureFreedom and me and my wife tested the various prototypes for them. It is the best trolley we have used - my wife had no problem using it and found it manouverable and managed to lift it in/out of van. She's only about 8stone so i cant see any blokes struggling (unless your wimps
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We had no problems with priming - the way they have linked the feed connectors means it draws from both barrells at the same time. At the end of the pipes are non-return valves which means the pipes dont empty every time you take the barrells out.
You cant see from the picture on the website (it doesnt do it justice), but the frame folds over so it takes up less space and is easy to lift etc.
There is also 10m of hose on the trolley which is great-you can leave the trolley at the front of the house if access is difficult-you dont need to get it past all those wheely bins etc.
Also you cant see from the pic but theres a pole holder on the side which made it easy, and the small wheels at the fron was great for getting up & down kerbs and moving around gardens etc.