Koh Samui or Krabi?

Koh Tao is more expensive than Krabi from what Ive seen. So is Samui.

Much much more expensive. hope you have a full bank account.

Plenty of transport from Samui to Tao. Note that Phangan between the two is a much better budget travellers' island than Samui. Got a beach for everyone from ragers to chillers.

Small point - that area is likely to be a bit drier than the Andaman, although as already pointed out May is usually not too bad at the latter. Nevertheless May is usually at or getting towards the start of the Andaman's wet season - around Phangan/Tao/Samui May is well short of the usual October into Jan wetter season.