I've tried both on at the Omega boutique where I live. It's a really tough call: 30 or 34 mm. They both look great. Both are very substantial and, while I originally thought I would want the 34 mm, I ended up liking the 30 mm on me better. The 30 mm is still a substantial presence on the wrist and the 34 mm was heavy. The 30 mm also comes with a silver dial and the 34 mm with a white dial. Both SA's liked the white dial of the 34 mm slightly better. I thought they both looked good, though the white dial has a fresher feel to it.
I ended up getting neither. Omega would not budge on the price at all and for that amount of money I bought a Rolex. I still love the Aqua Terra though so, maybe one day, I'll find a nice pre-loved one or I'll buy the quartz model before they disappear. Pretty sure they're being discontinued by Omega.