I haven't done both, but I have done the day pass at the British Colonial, and I've done excursions similar to the Blue Lagoon. So hopefully this will help!
Thoughts on Hilton: We loved our day at the In fact we're planning on doing it again this year, it really was perfect for us.
They had sand toys for our daughter to play with and the beach was incredibly calm, so she could splash around without me being nervous (she was 2.5 at the time). And it was with walking distance of the ship (about a 10 minute walk). Could not have been easier.
It seems a little pricey, but the cost also includes a credit towards food/beverages. So once you account for that, it feels more reasonable.
British Colonial vs Blue Lagoon: I think your choice will largely depend on what you're looking for. I think for older kids or adults the Blue Lagoon option would be better since they have the kayaking and I would assume better snorkeling. I think BL also has the dolphin and sea lion encounters.
Our kids are still pretty little, so I prefer to keep it low key and close to the ship. That way if they tire out, or start to completely lose it, we can go back to the ship and regroup if necessary. The British Colonial has less to do (really just a strip of beach, with loungers, umbrellas, wait servers, and a few sand toys), but it's also cheaper $60 for an adult and $30 for kids).
If all you want is a to lounge on a beach, splash in the ocean for a bit, and have someone bring you frozen drinks all day, the British Colonial will more than suit your needs. If you want a little more action, than I'd say Blue Lagoon is probably you're better bet. Hope this helps!