Both TGA and TIFF are lossless, that is they should look identical in quality unless something is wrong. TGA alpha support was broken in Photoshop 7, but was later fixed in a patch. Use whatever works for you, both will result in fairly large images. TIFF supports LZW, ZIP and JPG compression (with LZW and ZIP being lossless and JPG being lossy).

Test sizes:
Original image 1536x2048 pixels.

TGA uncompressed: 9mb
TIFF uncompressed: 9.01mb

TGA RLE compressed: 8.16mb
TIFF LZW compressed: 4.13mb
TIFF ZIP compressed: 3.97mb

So obviously TIFF creates smaller images, but your milage may vary depending on image content. TGA is also ALOT faster to store, requiring almost no CPU time for the compression. LZW is slower, and ZIP is the slowest. You'll have to weight the pro's and the con's.

Cheers.