As the above replies have suggested, the only real criteria is the blade you choose, rather than the machine itself.
Having said that however, having done a fair bit of metal cutting on scroll saws myself, my opinion is STRONGLY in favour of a machine which has variable speed - I find that running at almost the slowest speed available makes the job much easier and the resulting cut much more precise.
Regarding blade choice, IMO it doesn't matter so much if you use pinned or pin less blades, UNLESS you need to cut some tight curves/corners, in which case, just like when cutting wood, pin less blades are generally easier to get tight/small radius curves with.
Some time ago I published a "tutorial" about cutting sheet metal in the General Metalworking section of this Forum. That included at least 1 chart from a manufacturer of scroll saw blades specifically for cutting metal. I don't think it's possible on this Forum to have 2 separate pages open at once so I'll post this now, then come back to this post with a PS in a few moments, once I've found the link to that metal blade chart/s.
Hope that helps (and BTW, not blowing my own trumpet or anything - really - but you may find a read through all of that "Tutorial" helpful if you're going to be doing much sheet metal cutting).
Edit for P.S: here you go, linkies:
1. Scroll Saw Metal Blades Chart:
download/file.php?id=65768&mode=view
(Note: This chart is from Pegas, which I like, but there are several other good manufacturers too. The UK agent/distributor for Pegas is Axminster Tools).
2. Start of complete "Hacksaws - How to choose 'em & how to use 'em" Tutorial:
hacksaws-how-to-choose-em-how-to-use-em-other-stuff-t109936.html
HTH.