My Parents have the TwinAir engine in their 500 Lounge, which I've driven a lot and I also test drove the Punto TwinAir, in the older 'TwinAir' trim level (now deleted) earlier this year.
I found the engine less harsh in the Punto than the 500. Power delivery seemed more progressive - possibly it was slower because the Punto is heavier - the 500 goes like a scolded cat in normal mode! However, even keeping the Punto in 'Eco' mode didn't cause it to struggle & the engine seemed to quieten down quite nicely in 6th on my (albeit short) stint on the Motorway. Fuel economy in the 500 is mid-late 40s to the gallon, so I wouldn't expect more than mid 40's out of the Punto, but it does have a 6th gear that the 500 TwinAir doesn't - this might make a difference.
The ride was also better than I thought it'd be in the Punto, though like most Puntos before it, it does struggle to settle down on our uneven roads. The car I drove had the standard 'TwinAir' wheels which I think were 15" - having driven Alfa MiTo's (based on the Punto) with 16" & 17" wheels, the ride can differ quite a lot between wheel sizes - I wouldn't go over 16" as the 17"s in my Sisters MiTo (also a TwinAir) make it very bumpy.
GBT & Easy are very similar trims, but GBT adds 16" darker alloy wheels, body coloured rear spoiler (pretty subtle) & side skirts, tinted rear windows & sports suspension. The GBT is more of a looker than the Easy in my eyes, and would suit the new Junglist Orange better than the Easy. However, you might wanna test drive a GBT & Easy to see how much firmer the Sports Suspension is on the GBT, as this is one thing I haven't experienced - as far as I know the TwinAir trim I drove had standard suspension (and smaller wheels).
Best advice I can give is to drive both trims. Not necessarily with the TwinAir engine as I doubt dealers will have both, but even a 1.4 GBT will give you an idea how the ride will be. The plus point with the Punto is that if the TwinAir engine's no good, the new 1.3 85bhp Multijet diesel and 1.4 MultiAir 105 petrol are other decent engines.