I would say the best bet is to choose Ko Samui if you are going during the months May - Oct and probably Phuket the rest of the year as the weather patterns vary between both islands.

Hi Bernie
There is a huge range of prices in HK. Have you a budget in mind? For instance I would recommend the Excelsior as we were there a few years ago- however, we got a bit of a shock when we got their rates for our last holiday.
I would go for Koh Samui as well for the time of year you want. Do you want a busy place or somewhere a bit quieter?

we would look at 3 star in HK, and we would like to be in Kowloon or near Nathan road and probabky will go to Koh Samui would like somewhere near places to eat and not have to take taxis all the time.
we would look at a budget of around £1300 per person for the whole adventure

Bernie

If you haven't been to HK before, my advice is to not be too set on which area you research as long as it is close to the MTR you are only minutes away from most. Check BA for good deals.
Best to post on the Thailand forum for ideas for KS. Chaweng is the busy place there- too busy for me although we did love the Chaweng Regent where we stayed. Bophut is 5-10 mins ride away from there but has Fishermans Village which is a long road with lovely restaurants and bars. We found we only went into Chaweng a couple of times in a fortnight when we stayed there.