It's hard to really compare the two accurately since neither of them has a strong presence in the states as far as exposure goes. NXT airs in the UK and WWE is currently working to secure a US television deal for it this fall. As for OVW, it's completely without a deal of any kind as far as I know.
I have caught NXT on YouTube at times and what I've seen has been pretty damn good and gives me some idea of the overall feel WWE could have once Triple H ultimately takes over. Based on what I've seen of NXT & OVW talents, I have to go with NXT easily. The wrestlers TNA has brought up from OVW to Gut Check, with the exception of a few, haven't impressed me very much. Granted, most of the Gut Check guys are only on television for a few weeks before heading back down to OVW but TNA has booked the concept for wrestlers to be viewed by fans based on their first impressions. Based on said first impressions, I'm just not impressed by the vast majority of what I've seen. Joey Ryan had potential but he's little more than a comedy jobber now using the Val Venis gimmick with the sleaze factor turned up a few notches. Christian York's not going to be a major star due to his age and the guy's kind of bland & generic babyface. Alex Silva just flat out wasn't ready at all. Jay Bradley puts me in mind of Wade Barrett both in terms of style and look, which makes me interesting to see him. However, when I look at them compared to guys like Rollins, Reigns, Ambrose, Cesaro & Bray Wyatt; NXT just looks like it's on far bigger and better level than OVW.