Interesting stats there on render times.

PNG certainly leaves you with a transparent background (if you need it). But if you isolated the image in Carrara (alpha channel or however) and opened it in PS and opened the Vue image in Photoshop, just move it over then. Or save the Carrara image as a PSD file for future use. I'd leave it in PSD format before I'd convert to PNG. I don't know about TGA. I rarely use it. CIV 5 screen captures are save in TGA format...LOL. About the only time I use TGA.