I've seen some sigs made with GIMP, and others made with Photoshop. Having never even seen GIMP, I'm basing my vote on the sigs I've seen made with both programs =/ And Photoshop seems godly compared to GIMP.
PS of coz but i tried GIMP once it was too confusing for me, and btw you cant afford PS? let BT help you my friend..
I'd have to say Photoshop because i heard GIMP wasnt that great compared to photoshop but i never tried GIMp