I guess it depends on what you want to use it for more...but I'd personally go with the sugar gun 
I bought the pasta attachment for my mixer because I wanted to make pasta and use it on fondant. Get the cheaper one if they are both equal 
not to sound dumb but what is the sugar gun used for? I have an artisan stand and 600 both KA and have never heard of sugar gun, sounds interesting....
I work almost exclusively with fondant and couldn't live without my pasta machine attachment. You can use it to flatten paste to the perfect width for almost all cutters, including Patchwork cutters, plungers, ribbons, etc. I use my clay guns from time to time, but find the pasta attachment absolutely vital to my work.
Mine is a KA set with 3 attachments. The wide one, which is the one I use the most, the fettucine width one, which is great for ribbons and smaller sized bows, and the spaghetti width one, which is awesome for making tassles. I even made lace with the wide one - running a piece of real lace layered over a piece of gumpaste/fondant mix to get the impression.
I couldn't live without mine. It's a bigger investment at $99 (amazon.com has the best price) but you get more bang for your buck. The clay gun extruder is $36 (I found the cheapest price at Beryls.com - but definitely shop around).
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