[gonzo]

There's plenty of sightseeing to do in BKK alone for a first trip I'd have said. You can always travel to a few different places on your next trip. You're already spending a fair chunk of time in Ubon (presumably for personal reasons as even on a 30th trip I still don't think it would ever hit anyones top 20 places in LOS to visit list!) which will seem very different to BKK or Pattaya anyway.

More time in BKK or Pattaya will give you less travel hassle/costs and more time to enjoy yourself with still enough sightseeing for a single trip.