In BKK I'd stick to a hotel on lower sukhumvit, plenty of good ones to choose from depending on your budget. Pattaya is cheap and easy to get around so location isn't so important there IMHO.
I'd still recommend Koh Samed for a side trip, its a really nice chilled out island which makes a good contrast to Pattaya which can get a bit hectic. It is very quiet though but as long as you're taking company with you, you'll have a great time.
Pattaya is all about the nightlife so I've never really found that much else essential in the way of sightseeing there. The Sanctuary of Truth in Naklua is worth a visit (
http://www.sanctuaryoftruth.com/) but thats about all that comes mind.
The infamous Soi 6 is a good way to spend the day but thats more of a must-do than a must-see
