Singer T&S 603E or 626?
One of each sitting at Goodwill,,
Do they both have all metal gears ?
does the 603E do a zig zag ??
Not personally familiar with either. A brief search indicated the 603e is a slant shank machine, something I am not fond of, but my reasons are due to the heavier weight material I have used in the past.
It also appears there is a straight stitch version with cams or a 30 stitch machine with a dial. If you can't easily find a zig zag setting, it's probably cam driven. I am not opposed to cams if they are all there or available.
It appears the 626 is also a slant shank. I think most or all of the touch n sew machines were. It appears to have a zigzag stitch.
Do a little search for parts, manuals, hooks, needle plates, etc on eBay and other sites to see which has parts available.
If both have parts available and seem to be in decent shape, I would lean toward the 626 if the 603 is cam driven, only because you will have to change cams to change stitches.