I have no idea whether SWA would go after used -300s or ATA's used -800s, but there are pros/cons to each.
More available than used NG
Older, thus more MX
Cockpit commonality issues
Need to install HUDs
Only 3 F/As required
ATA's used -800s
33 aircraft, more expensive to acquire
Oldest aircraft 3.5 years old, less MX
Easy cockpit commonality via software changes
Already have winglets, more fuel efficient.
Already have HUDs (a slightly newer model than SWA's)
Already have big engines, 27,000 lbs thrust each
Need 4th F/A
There's always the possibility that any acquisition of used aircraft (-300s, -800s, whatever) could be for a fixed 2-3 year term until Boeing can deliver new "standard -7H4" models new to SWA.
[Edited 2004-11-10 01:40:56]
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.