I have used both and experienced great service both times. I preferred We Go Shop only because it was obvious she tried to get us good deals...we asked for 2 regular size boxes of cereal as an example, and she bought us a giant club pack instead for much cheaper (it was on sale). They did a great job, and I'll be using them again on our next trip for sure!
The only downside was that we did order beer, and our flight was delayed for two hours. I forgot to call her until last minute - right before she did our drop off. It was easy enough to pay by credit card over the phone so she could leave our groceries at Bell Services, but she needed ID for the beer and if we couldn't provide it she would've had to take it back (which would have devastated our friend who we got the beer for - he was REALLY looking forward to drinks on the balcony at night!). We finally got the idea to take a picture of my ID on my smart phone, along with a picture of me proving that it was my ID and that worked. WHEW, crisis averted!
She was great about the whole situation though, very accommodating.