1 day was enough for me in LP, but a big thumbs up for the rest of Laos. In LP I found it a tourist enclave. For example, even finding Laotian food was quite difficult in the city. The curfew and having to roll in so early in the evening rubbed me the wrong way too. I also found LP rooms quite overpriced, but at least when I spent money, I did manage to find quite a clean room.

If you're looking for very laid back and did want a beach, I might cross off Krabi and Phuket and go for some of the smaller beach destinations here in Thailand. But it's a matter of taste. The two choices you present are at both ends of the spectrum. If possible, why not try them both. One last option, what about Angor Wat?