I pretty much covered all of this in another thread yesterday, and as I really don't feel like reiterating all of it, copy and paste time...
I know that this seems to go against how a lot of the "older" collectors think, but I've always felt as if you're limiting yourself too much if you only collect one line. There's so many varying lines and figures, all with ups and downs to them that it would, for me, really suck to miss out on them. IF all I collected were Bandais, I'd miss out on SO many great figures and the breadth of the whole market.
Going with the Bandai's again, I would hate to limit myself to the dull, unimpressive Bandai Megalon that they don't seem interested in updating. There's alot of other examples I could give in that regard, however, I would also hate to miss out on their 8" GMK Goji as it's a truly great figure. Same goes for the GMK Ghidorah, the Memorial Box G 84, and many others. But, again, if those were all I limited myself to then I'd miss out on the high-quality and, honestly, stunning figures that Marmit releases. I hate to think of my collection missing my Marmit Burning Goji or Titanosaurus, which I think best the Bandai's. But if all I went with were Marmit, then I'd miss out on the amazing M1 Megaguirus or the M1 2nd release Manda, which I think are two of the best representations of those kaiju in vinyl you can find. Plus my M1 Frankenstein...? Try to find that from Bandai.
And all of those are great, but what would I do without my Revoltech kaiju? God, I love them. Their Anguirus destroys practically any vinyl version of him that's ever been released, as far as I've seen. Baragon? Stellar. Rodan? Amazingly accurate sculpt and paint for the price, regardless of how you feel about the articulation (which I have no problem with). Mothra? Well...Mothra is Mothra, haha, love her or hate her. Buuuuuttt, if I only stuck to Revolt, I'd miss out on the Monsterarts Goji, and that would just seem like a crime.
Basically, I think you can see where I'm going with this: Collect the figures YOU like the most. Don't be beholden to some unacheiveable "perfect" collection. You won't ever be able to have all of the Toho kaiju in one line and in one scale anyhow. As I said before, you'd have to dip out of Bandai's if you wanted a Frankenstein figure, so you gotta think about that.
I began collecting figures, believe it or not, just under a year ago. I've spent a lot of money, frustration, worry, and personal time hunting and scouring the nether-web for the figures I have and I'm pleased as punch with every. Single. One. I love them all and feel a real sense of accomplishment at what I've built, so having said that, I don't really care if any of the "seasoned" collectors think that I don't have the "best" version of a figure or really, what they think at all about it. That wasn't why I started doing this to begin with, so it's a non-issue for me.
I started this whole crazy trip with one goal in mind: A representation, at least one, of EVERY Toho kaiju and mecha character/vehicle. Scale does not matter, since I would NEVER be able to get all of them in scale to each other. Line or style does not matter as I like super-realistic figures AND vintage styled figures equally. All I want is to have one version, at least, of each of the monsters and mecha. That's it.
It definitely seemed like an easier, cheaper, and, well, saner idea than, say, becoming the next Legion1979.
But that's me, and if scale, line, etc. matter to you (and as a collector of many items outside of toys, I can understand why you would want everything in a series) then I can't help you with what I've said. I just think it's sooo much easier to be open minded about this stuff. Get what YOU want, make yourself happy.
I'll tell you though, that Iwakura mini Gorosaurus just ain't going to cut it for too much longer with me...
EDIT: Oh, and I only have one American figure and that would be the 2nd Bandai Creations Godzilla 54...'cause it's better than the Japanese Bandai