A lot of collecters are completists...in other words, they must not only have every Bandai (or insert Marmit, Bullmark, M1, etc) figure, but also every different color variation of each figure release. Not that I am knocking that or saying that's a bad thing...to each their own.
As for myself, I tend to collect what I like (sounds cliche, but it's true), regardless if it's Bandai, Marmit, Gigabrain, M1, or even Trendmasters. Usually, I focus on figures in the 8-10 inch range (I absolutely cannot stand the Bandai 6 inch line...bring back the 8 inch line damn it!!!

). I prefer figures that scale fairly accurately with each other...doesn't have to perfect. As long as I like a figure's interpretation of a particular monster and feels it captures that monster's look and spirit, then I'll buy it....as I said, I have absolutely no alignments with a particular toy company. As a result, my collection is really a "mish-mash" of Bandai 8 inch figs, Marmit Monster Heaven Figures, 8-10 M1 figures, a few GigaBrains, and even the occasional Marusan and Trendmaster figure...all them usually scaling very well with each other. Most completists would be aghast at the random-ness, but I for one am extremely proud of my collection and wouldn't trade it with anyone else's.
I also don't keep the tags on them or keep them in their bags/boxes. Some collectors buy a figure, often to never even touch it or remove it from it's packaging in fear it will hurt its "collector's value" later on. I try not to knock other people's collection habits, but I don't really don't like this approach/philosophy at all...I find it a little silly and prudish. One should enjoy their collection and not do it just for the hope of reselling them down the road when their value increases, but that's just me.
I have never gotten into the Imperials/Bullmarks (mostly due to price range and their often inaccurate interpretations of the monsters). But I think it's really cool that some collector's have them, because these toys are basically like stepping back in time as the earliest G figures released and have great historical value which you can't put a price on, which I think is awesome. Maybe one day when I become more financially inclined, I might give collecting vintage toys a try someday...
I don't collect the model kits, because I suck at putting models together, and I have a very bad habit of not finishing what I start.
In the future, I am considering on starting a collection of Gamera figures, as I have virtually no figures of the shelled one or his enemies. And in Nakano Broadway and Akihabara districts of Tokyo, there are tons of really great Gamera figures (even the supposebly "rare" 8 inch Guiron figure is usually pretty easy to find). One of the reasons I never got into collecting Gamera figures was because after the 1990s Gamera films, all of the vinyl releases, regardless of scale or company, look
*way too similar* to each other and lacked a certain appeal/charm to me as result.