I have and love 'em both. If the rumors of Green Valley being soon discontinued are true, I'd recommend getting it first. I'll forever kick myself for loafing on Vintage Tabarome.
Not taking anything away from Green Irish Tweed, but Green Valley is really an underrated masterpiece. One of the best upscale green/spring scents. It smells really expensive and refined. They both do, really, but I think you'd be bummed out if you one day decide you wanted Green Valley only to discover that it was hard to find. Also, I really dig the drydown of Green Valley-- it has a warm patchouli/sandalwood/oakmoss base that gets deeper as it goes. I also think Green Valley is a very safe blind buy-- I can't picture many people disliking it.