With Shelter after previously volunteering with Oxfam. Have to say that the Shelter offering is infinitely better (only in my opinion of course!) for the following reasons:
Campsite is closer - no long trudge up Muddy lane last thing at night.
Food - more vouchers and better maintained tea/coffee
Showers - no limits to the number unlike Oxfam and 10 minute queues max.
Shift patterns - no overnighter or early starts. Always finished by 3am and never started before 11am so didn't have to really rain in the night beforehand.
Always busy - those long 8hours on a random road crossing with Oxfam compared to ours shifts which flew by.
Better fun - we were on Stonebridge which was a massive party - crew bar dancing along with the punters, etc and Fat Boy Slim and Roger Sanchez playing in your tent - what could be better than being in the middle of a party!
Disadvantages vrs Oxfam? Struggling really. You can drive onto site with Oxfam rather than have to catch the bus. This way you can take more stuff and leave when you want I suppose. Aside from that ....we'll be back with Shelter next year!