Depends on the quality of welds you need. If you want to weld tanks or thin AL sheetmetal, then by all means, learn tig. If you just want to weld this and that misc aluminum stock, a spool gun works great. I have a Ready Welder II that I have hooked up to my Millermatic 185 (DC) and it works great for some stuff, not great for others (basically a spool gun). Most the time it works just fine. You will need another bottle of gas (pure Argon), but the regulator is the same. What are you intending to weld?
FYI--the Spool gun is way quicker than typical TIG. That said, TIG is next on my list to learn and get a new welder. They are pricey. FYI--not all tig welders do aluminum...