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Don't expect to do a whole lot of moving with the Vulcan, shit will jam if you look at it too hard. Although, if you were willing to invest in modifying it, then you'd be incredibly intimidating (it's already a monster as it is).
However, the Longshot can be easily modified to kick serious ass as it is, so it boils down to how much modifying you want to do and how much you want to spend. I'd personally go with the Longshot because I prefer hitting things as opposed to hitting everything around them.