Having stayed at both (in suites), the Hyatt Capital Gate is a fine property for an overnight stay. New. Modern. Beautiful, well appointed large suites.

However, the PH Dubai is where I'd spend a relaxing 3 days. It's a beautiful oasis. You will certainly have a wonderful time, even more so if you can secure a DSU. No questions about it.

I also feel there is more to see / do in Dubai, but certainly a few gems in Abu Dhabi as well if you can spend a day / night there, if the additional cost makes sense.