IIDX really is the king of music games.
Pop'n is fun, no doubt, but I can't say I think its as good. Much of the music in it is terrible (with some gems strewn about), and its a bit easier (scoring and song difficulty) than IIDX.
Who plays pop'n with their fingers? (Besides those with mini-controllers)
The transition between mini controller and ASC isn't that big, really. I can adjust between them pretty quickly. Thus, the mini controller is fine.
So what do you guys think 7th style ok to jump into as a newbie
9th has the slickest interface, and that might be more important than you think. The songlist isn't the easiest, though, nor is the Beginner's Mode.
Most say 5th has the easiest Beginners'. I started with 6th and was fine, largely due to Drill Mode (which is only in 5th and 6th). 8th is good, too, and 7th isn't a bad style... They tend to get harder each release.
Hell, you can buy pretty much any style to get into, I suppose, but I suggest starting with 6th or above. Quick random notes:
3rd - Ancient (yet cool-looking) interface; doesn't save doubles scores seperately (overwriting standard scores, gets confusing), doesn't notify you when you get a high score, doesn't show you song letter grade after playing. Has the easiest songlist, overall, though it lacks a Beginners mode (which scales every song down to three stars or less). Hi-Speed 3 is hell, and largely unusable to new players. You'll care about that sooner or later.
4th - Interface-wise, a slight step up from 3rd. Includes a display when you get a new score, saves doubles scores, has a useful new display option for doubles players. Has a small songlist. Supposedly the rarest game. I'd suggest it as the last style you buy, to be honest, though it does have its own awesome songs. Hi-Speed 3 still ass.
5th - Still no individual song letter grades. Hi-Speed 3 slowed down to largely usable, though some songs make you want a faster alternative. Beginners' Mode added (1-3 star variants of every song), Drill Mode added (quick sections of songs, more of a tutorial than anything else, I found it very useful in 6th). Songlist I'm a bit undecided on, though its one of the smaller styles.
6th - Pretty as hell. Great Beginners' and Drill. Cool unlockable videos of good players. The series starts to get pretty trance-y from here on out. Nice high-difficulty songs. I started with this style, unless you count emulated Wonderswan Beatmania and BM98 on the PC.

Hi-4 added, wonderful.
7th - The one that wasn't supposed to come out. Arguably the style with the best soundtrack. Drill mode removed. Don't expect to unlock the last song in the style, Abyss Heavens. Its a preview track from 9th, though.
8th - No real interface changes from 7th. The stupid: Expert Mode plays of songs don't save scores.
9th - Hi-res. Biggest collection of difficult songs. Not many old songs are included in this release. Some people feel this game is sorta the beginning of a decline in music quality, but I don't really agree with that. Not sure if I can suggest this style's beginner mode.