Forgot to add.. On the "space side" .. They are very very close... Be that seat width, or leg room.. However that might change if you're in an exit row, however on AirAsia that's a hot seat, thus a fee applies.. On Thai Smile, there is none, however they are assigned at their discretion.
Service wise... Also close.. Having flown both numerous times my take-away is that Smile "feels" more like a regular "full fare" carrier.. AirAsia tends to have a more LCC feel to it..
That's not to say one is better than the other, but is how I perceive their overall product to feel.
One of the ways I get this feel is that on AtrAsia, no food drink is free - its all chargeable .. On Smile a basic cold snack is free... So the cabin crew on Smile still do one "classic" snack service without a fee component for each passenger.
This is, to me, pretty much the only notable service differentiation between them.
Overall service to me is good on both.. However since you're talking people, there's always an element of luck as to the actual group of crew you get. On both you can get fantastic as well as mediocre.
In summary.. To me given the rather short duration, id look at the math first and foremost..once you've done that, then I would add in a hot seat exit row on Air Asia and then see how the two compare..
At this point (assuming you don't or can't get an exit row on Smile for free) you'll have a slightly "better" seat on AirAsia (better defined as more leg room) than Smile.. Now see how the costs add up, as ostensibly you'd have a "better" seat on one than the other.
**!Note of disclosed: my spouse is a Senior Flight Attendant for ThaiAirAsia. Therefore via her employment I have unlimited pass travel on all AirAsia lines. I have pass travel on Thai Airqaya, to onclude all Smile sectors, due to my own employment but receive no direct personal remuneration from either carrier nor subsidary
Edited: 1:13 am, April 29, 2013