6 months with hand luggage?

Sure,if you are happy with checking in luggage,why not?

Just try not to carry too much weight ;-)You will regret it when you have it on your back.

Lighter bags means more comfort,less money spent,better safety too (you can keep a small bag near on transport for example,instead of putting it on top or under the vehicle.

Pack whichever bag you choose,carry it around for a few hours and see how it feels....I often see people staggering around with monster bags they can barely lift off the ground ;-)

May be easier not to have to worry about 100mil max and 7kg baggage allowance etc for carry on?

Exactly it's just unnecessary hassle. You've already been to Thailand so you have a good idea of what it's like. The SEA trail is mainly backpacking in name only, it's mainly a few steps to or from a ferry, taxi, bus etc there is very little actually carrying for long distances.

I remember when I went to Thailand I was gutted as I could have bought home bags and bags of stuff

That never changes !!

If you decide on the smaller bag,and want an ultralight weight daypack too...Sea to Summit make some good quality ones.

They are not suitable for trekking.Not enough support.But fine for light walking around a city,going to the beach or things like that.And easy to roll up into a little ball to carry in your main bag,or in your pocket.

Thanks for being so helpful :-) I’ll check them out now x

Ok,you're welcome!

I’ve had a look and AirAsia charge around £15 for checked in luggage

Sure,if you are happy with checking in luggage,why not?

People like to perpetuate myths but not all low cost carriers are out to fleece you, several do have a free luggage allowance.

Virtually all airlines always check luggage weight to facilitate fuel calculations. Some will let you slide in terms of over weight but Air Asia is usually pretty strict and will charge extra if you're over. And with them, buying over weight allowance in advance is MUCH cheaper than at the check in counter.

I've taken Airasia perhaps 50-60 times in total,and I don't think they have ever weighed my carry on bag.

Perhaps if it looks well over the size/weight limit,they might do so....

I have always taken the view that I would rather have a half empty bigger rucksak than one that is too small

The big international carriers seem very lax about what they let people carry on board but may of the low cost "local" carriers are far more strict. I tend to avoid the low cost airlines where the costs escalate in front of your eyes - in Thailand I use Thai Smile (just make sure you book them on the Thai Smile site and not the Thai Airways site) or the train as the cost of taking my luggage with Air Asia was nearly as much as taking the train