Selecting a song brings up the difficulty/modifier selection menu. Pretty much all the modifiers from ITG are in here too. One important change Pump players might want to know about are the noteskins - by default, the arrows are colored by 1/4, 1/8, 1/16, etc. Of course in old Pump versions you had orange for UL and UR, yellow for Center, blue for DL and DR. If you prefer that format, fear not! Select the "Classic" noteskin and you're good to go.
Speed modifier wise, it seems that they're faster than what the same mod would be on a previous Pump game... i.e. x1 feels more like x1.5 on older Pump mixes. I ended up playing most of my songs on x1.5 because x2 often felt way too fast... but maybe that's because the pads were pushed up pretty close to the screen. So I would highly advise against picking x3 on anything if it's your first time playing this version. Also, the available speed mods for selection were (this might not be 100% right): x0.25, x0.5, x0.75, x1, x1.5, x2, x3, x4, x8, C200, C400, Random. Yeah, those are two pretty horrible choices for C mods.
(P.S. I thought Aliens In Our Midst was really fun, yes it's very slow and hand-heavy, but I enjoy hands and much prefer to play songs that don't have 8 bazillion steps)