Ive picked up some Alien cans when they didnt have the color i needed in 94s. real nice color and control. The pressure system on the valve varies depending on how hard you press the cap. So the harder you press it the more paint. I believe hardcore is fixed pressure (never used it, but pretty positive)
well what are u using it for for bombing get the hardcore as its a higher pressure=more paint comes out for piecing get the alien can as it is lower pressure=less paint and thinner line(also it has nicer colors)
if you're piecing i'd go with alien. but never use alien for bombing it's way to low pressure for that unless u wanna be there for a half hour doing 1 throwie. mtn hardcore can be used for both but hardcore is best for bombing out of the 2. mtn hardcore is also good for piecing.
i like the alien mini cans, but i feel like montana hardcore is way more reliable if your buying normal sized cans.