When we went, we got the "Express" (when it was still QBOT). We had Merlin passes at the time, so if I remember correctly we got a discount on them. I would say it's worth doing if it's a one-off visit, since although it costs a bit more, you'll be maximising the value of your entry fee.
However, we weren't impressed with Legoland in general, and even as massive Lego fans, I doubt we would go again. All the rides seemed very dirty and "tired", and there wasn't really anything that stuck out to us as "yeah, that was really awesome". We went to Thorpe Park the next day, and we all enjoyed that a lot more (even our boy who was 6 at the time).
Glad we paid for the QBots so we could go on more of the rides, but if our entry wasn't free in the first place then I think we would have been very disappointed.