Hi SadCanadian.If you're looking to experiance the naughty nightlife,Krabi and Siem Reap are probably not the best choices.If you're going for temples,Siem Reap is the place but if you're looking for nightlife Phnom Penh is definately the better choice.
I paid 12300 baht(360 cdn) to fly Bangkok air... BKK to PP and SR back to BKK.It's a 40 minute flight to SR from BKK so you don't waste a lot of time getting there.How much money are you saving?
As for hotels in Siem Reap, Golden Temple Villa was recomended to me.It's around $20 US was full when I checked it out but looked good for the price.Bopha Angkor was around $50 US and to me the rooms looked quite nice.(also full so reserve ahead)Both have websites.
I haven't been to Krabi,but for nightlife I understand it's quiet.I'm probably stating the obvious but Patong Beach on Phuket has a rocking nightlife,lots of diveshops and nearby divesites and the Mai Thai Bar where you will definately feel welcome.Scuba Cat on Beach road will teach you an Open Water course with one day's advance notice.
I don't know,maybe you are just looking for a quiet place to reflect and maybe find one girl and that's why you chose Krabi.
In Bangkok,Gullivers is popular with travellers and expats.BM's can sometimes be found there when in town.
Cheers
and Happy Holidays
PS If you're going to Thailand in February,I think you should change your name to LuckyCanadian!!