A little bit of everything.
In a prior life when it was 1990-ish and I was able to just drive down to my local multi-tabled shop for games any day of the week, I'd say friendly gaming (on the somewhat more competitive side) was my main draw, although I found the setting fun and didn't dislike the hobby-bits either. That was super-fun, in a nostalgic sort of way one can never actually recreate. I'm sure it doesn't hurt that it's mixed in with my memories of driving for the first time. Haha.
Now though, I think the main draw is a mix of painting and competitive gaming.
With painting, I'm a bit better than I was as a child and can find more fun in it than stress. As the price of minis has gone up, I feel that treating armies as slow art projects makes it much easier to stomach the purchase cost than simply picking up everything at once too. It's hard (impossible, even) for me to justify buying a whole army at once so I can get games in. I'm not poor, but I'm not that sort of wealthy either. However, it's much easier for myself to justify spending $55 on a squad that I'll spend weeks trying to paint to a high standard.
When I say competitive gaming I mean gaming in a competitive setting more than constantly pursing the best army/list (as fairly infrequent tournaments are the only places I'm likely to get actual games in these days). I'd aim to bring decent lists and do my best, but not particularly expect to do great. My understanding is that a few tournaments use a sort of matching system based on day one performance, so I might end up playing against other people in my general boat on day two, which could be pretty fun. (That said, I've never actually been to one. For all I know, I might end up totally disappointed. But I live on the US East Coast, and am pretty close to the NOVA Open and a few others.) I'd love some good beer/pretzels friendly gaming ... but have no one to do this with these days.
My "local" GW is within drive-able distance still, but it's overcrowded and short on tables (and on the longer side of drive-able). People also seem to use facebook to arrange games with people they already know at it. I'm sure I could manage to do that in the future, but that lacks the appeal that true pick up games had for me in the 90's for various reasons. One of the things I liked most about 90's pickups was all the random people I met, who I never would have talked to otherwise. In that sense, tournaments might be closer to it now than a current face-book arranged game at my GW.
The background is nice for a setting, but I can think of several books I'm likely to go out and buy before I bought a Black library one just to get more of the setting. (This is one reason I don't love the recent trend of constant "story progression/rule" hybrid books that GW is pumping out).
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Anyways... still pondering a bit. But IG with allies seems like it's probably the right choice for me.
They're not my absolute favorite mini line, but I like them (I'd have prefered marines with the old tanks, but I simply can't make myself like the new marine tanks for reasons of purely personal taste). The guard give me access to painting and playing with allies I like even more. They (probably?) have as many playstyles as most people. I feel like sisters will certainly be much more limited in playstyle. Eldar might have more, but have far fewer ranges for me to paint/play with. Marines probably have as many playstyles, probably more based on chapter selection. But it simply feels more organic to me from a fluff/painting/ratio perspective to have marines/sisters/knights allied to guard than the other way around. Choosing IG feels like it (almost) lets me have my cake and eat it too.
... and I've never tried the "new' GW paints, but I hear good things about their ability to make painting things like skin (an old big issue for me) much easier. I'd sort of enjoy the ability to play with that in a non-helmeted list too. IG could be big beneficiaries of those paints, for me at least.
My only real worry is that after I finally finish painting up 120 cadians, they'll release new vastly improved cadians. Because I won't want to rebuy/repaint that much for sure
Thanks again, everyone. You've been really useful as a sounding board for me.
PS: One final question, if anyone knows. Do the Digital Deluxe Edition codicies GW sells auto-update to include FAQs and such? Or do those need to be hunted down seperately still? If they auto-update I'm totally buying one. If not, I think I still prefer a print codex.