RE: GP 1/4 scale Piper Cub Conversion?
Two 3-cell LiPoly packs in what configuration? 3 in series or 6 in series? Volts are the electric equivalent of cubic inches and the number of cells in series determines Volts. A 3-in-series, or 3S LiPoly is more suitable for smaller airplanes, say, in the .25 size range. 6-in-series, or 6S, would be more suitable to larger airplanes in the .60-size range.
For .40-size airplanes, I would recommend a 4S or 5S LiPoly, then enough packs in parallel to handle the current, and/or give you the duration you want. Of the two motors, I would recommend the AXI because it can handle more power. That way, if you feel you don't have enough power, you can get more by adding a bigger prop or by going to a higher-voltage battery. With the Jeti, you'll have maxxed it out.