I haven't used the Mana, but I ran in the previous version of the LT which is a similar build without the posting. My brother has had a few pairs of the double posted Mana too, so I've seen the shoe. I do have the Rocket, and have put about 200 miles on it so far. I also have the Ace and Adios.
First, I would ask why you don't just stick with the current Mana. Running Warehouse still has a bunch of pairs and you could stock up if you don't think you'll like the new one. Second, I don't think that the new one will be vastly more bulky than the old one (if any), even if it looks so to you.
If you decide to try the Rocket, here's my impressions:
1. The midsole is denser than most flats, and with the rubber outsole it should last a long time but not with super cushioning.
2. The heel is substantially lower than the Mana/LT/Ace/Adios and I notice it quite a bit. I feel like it's lower than the old RC.
3. Not sure if it's just me, but it doesn't feel like it gives my legs as much "pep" as other shoes. It doesn't seem to slow me down though.
As far as the Ace/Adios, I don't personally like the plastic either, which the Adios has. The forefoot of the Ace is actually all rubber, although the tri-pronged design leaves gaps on one panel and seems to wear out a bit quicker than full rubber and I started putting a layer of shoe goo on that panel which seems to work well. They both have a bigger heel than the Manas too, although I don't think they're heavier. If you don't mind a bit of extra cushioning, the Ace might be a way to go.