I have used both of them many times and had very good experiences, they are so much cheaper for sending large/heavy packages.
Parcel Monkey's customer service is email only, and they take about a day to respond, but they do sort out issues. I once scheduled a collection, but then realised that I needed to change it. After emailing them (about 3pm) I had a response at 9am the next morning saying it was rescheduled and sorted.
Just make sure you check the restricted items list and claims procedure for each first, because I know somebody who sent a computer monitor and it arrived damaged, but because it was on the exclusions list, he wasn't allowed to claim compensation.