Just go with the alien invasion and buy it directly from http://www.alienpb.com/, as it will be easier to maintain than the axe, and it will be very comparable as far as smoothness is concerned, and IMO I think it is a much better marker than the axe, though you can't go wrong with either, but if you truly want a stacked poppet the alien invasion is the way to go.
If dangerous power made a gun that was super good, like as good as an lv1 or something and was super reliable, I wouldn't buy it. Dangerous power needs a reboot like smart parts. Makes some better shit. Anyways, my dangerous power rant is over. Steer clear of the g5. axes are very reliable, only one thing to maintain (the bolt), shoots like a high end, very good efficiency, and all for less than a g5. Also the mini lacks MANY features such as the asa and micro switch. The trigger on the mini, to me, is so unimaginably bad, I can't stand to shoot it semi. Anyway axe>everything>mini>>>>>>>>>>>>>>>>>>>>>>>>>g5/dangerous power
I don't believe that last part is true, as an used dm9 or pm8 will be better than the axe, it should look like this: dm9>=pm8>vice>axe>mini>g5. The vice and the dm9/pm8 will come down to preference (spool vs poppet), but otherwise IMO they are better than the axe, but with everything it will come down to personal preference, as you can't go wrong with the dm9/pm8, vice, or the axe, I would suggest you try them out first if you can and see which one you like the best.
I would go with an used dye pm8, dm9, or a bob long vice. I personally would go with the dm9 or the pm8 as I like spools, but if you like poppets I would go with the vice. With these markers it will come to personal preference, so I would suggest you try them both out if you can.
As far as the axe, invasion, drone, etek, and egos are concerned, IMO the pm8 or dm9 and the vice are better markers, as in your price range the pm8 or dm8 will be the smoothest marker (besides maybe an used droid, which is good as well), and the vice will be the most efficient.
So it will depend which marker is best for you, if you like poppets go with the bob long vice, and if you like spools go with the dye pm8 or dm9.