Sorry to be the party pooper, but my experience of IB Caol Ilas is that they have been all over the map. A 1990/12 from Signatory's Un-Chillfiltered Collection was one of the most unpleasant bottles I've ever had, all harsh ashtray flavors. That doesn't mean the bottle you're looking at isn't a good one, but if you can't get a sample or a specific recommendation, and can't afford to be daring, you might want to stick with OB's, which are probably not the most stellar of Islays, but are at least fairly consistent. Remember that Caol Ila wasn't available as an OB until quite recently and was all intended to be Islay-influence in blends, allowing Lagavulin to bottle more as single malt. The wood seems to be highly variable, indicating that they would use whatever they could get. Cask selection, obviously, is everything. A Douglas Laing bottle I had a couple years ago was awesome, but another Laing recently was very disappointing, especially for a Laing--again, ashtray on the palate. The only bad Laing I've ever had.
In other words, yes, the best Caol Ilas are IB's, and so are the worst, and there is a wide range in between.
The 12 will certainly give you more of what you're looking for than the 18, although I think the 18 is a perfectly good dram. I also think the cask strength is your best bet, if you can get it--it's not available here, and maybe not in the US (anyone seen it?). I'm not sure why, as it is bottled in 750's for Canada.