The Great Planes Elecrifly site is short on real data, but it appears that the 600T is rated at 8-cell (9.6v max) and the one example they give - 8x4 prop - indicates something over 20A on 7-cells (140W). If that is about its limit then you would need to prop for only about 12A-14A max on 3s. Given that, either of those packs mentioned would be more than adequate, though of course the 20C pack (30A max) will hold better voltage under load than the 15C (22.5A max) pack.