I would suggest visiting Bangkok first, before visiting Singapore. Bangkok will stress your senses beyond belief, something like no where else in the world can do. Or try Phuket, Thailand instead of Bangkok to still get the culture but without the intense crime of Bangkok.

After your are saturated with Thailand, fly to Singapore and see the amazing sights of a beautiful oasis to say the least. The night zoo, architecture and home of the Singapore Sling are so much fun.

As others said, both places are like night and day with each having a persona of their own and both well worth visiting.