It is going to be totally based on personal preference. I know some very talented aggressive skiers that prefer a softer boot, whereas other may prefer a stiff boot. I do however have to disagree with the statement above regarding flex indexes of various companies. It is true that they are not directly comparable, but they are very very close to each other - with the exception of technicas flex index that was used until this season. I assume that most of them have something to do with distance resistance... but i have no confirmations on that. Essentially most manufacturers' boots line up with each other in flex. A 100 flex Salomon, is very close to a 100 flex Lange, or 100 Flex Technica, or 100 flex Nordica... etc. The list goes on but you get the idea. However, all of these boots fit very differently. You should be less concerned with flex and more concerned with fit. If a boot is going to be too stiff for you, you will know when you try it on.
Personally i like to race in a very stiff boot (Nordica Doberman Soft - 150 flex). My free skiing boots are Head S12's. While the head flex index does not compare to any other - it is about the flex of a nordica 100 - 120 (somewhere in that range - probably to the higher end 110+). To me the S12 is quite soft... I also found the Technica XT17 to be VERY soft and the XT24 to be moderately soft. The Lange RL1 and corresponding Rossignol boot are some of the stiffest i have had on my foot - more so than the Doberman because of the amount of forward lean.
Try on a few boots to find a fit that you like and then test out flex indexes. If you find a stiff boot that has a good fit, it can always be softened.