I've stayed there 8 or 9 times, I've stayed as Casa Mexicana downtown once. I've never stayed at Casa Martillo but from what I've seen online, you're comparing apples to oranges. Should make it an easy choice for you.
Casa del Mar is a hotel, Casa Martillo is a condo, so it'll have much more space, amenities, etc.
It's closer to town than Casa del Mar, which is nice for eating out at night. I think it was 80 pesos for a cab one way to town from Casa del Mar last February. Casa Martillo will feel much more private I'm guessing. It looks to be a big upgrade from Casa del Mar, judging by the pictures.
Casa del Mar is a nice little place, and one of the cheapest for air/hotel deals from where I live.
It has a restaurant, tennis courts, a pool that you can swim up to the bar, and a nice "fake beach" area across the street. They have a great dock and I've never been charged to use it to get on dive boats. There's some pretty easy snorkeling there, though it was really ravaged by Wilma. They have an onsite dive op, but I've never used them.
Long and short of it, if you're looking to save money and Casa del Mar is considerably less expensive, I'd stay there and you won't be dissappointed, at least I've never been disappointed.
If money isn't an issue, I'd probably try the Casa Martillo. It looks nice, is close to town and it probably won't disappoint either.
I wrote a review on this board back in March or April. I think the title was Casa del Mar and Deep Blue 1-24-09 to 2-7-09, or something like that. There's more about Casa del Mar in there.
-Blair