The first camp is the way I think most people, including myself collect armies, as optimized and inclusive of a certain point value. There may be some sort of "sideboard" but for the most part units are not bought that will not be used 90% of the time. I think this lends itself to the fact that 40k (at least in my area, Northern California) is mostly an 1850 game at the tournament level (and those that are not are within 20% of it). Most of us plan out our lists with tips online, vet them in forums, and talk them to death with our friends. For example, for my IG list I took the popular "leafblower" 2500 point Ard Boyz list and scaled it down to 1850. The only thing I changed after my initial purchase was I swapped the two Medusa's for a second manticore and added lascannon teams. For the most part I play it the exact same every time. The problem is that it leads to a bit of burnout. So I've decided with my CSM list I'm building I am going the road less traveled.
The second (and what I am doing with my CSM) is to just collect units that you like, that you want to paint and that may just look cool. This tends to lead to a large collection of models with some models not getting much play time.
This was how most people collected 40k when I first started, back in 2nd edition. Through my time up to the end of 3rd it was the way I collected my armies. There wasn't much planning, I just bought stuff I liked. If it didn't work I put it on the shelf and waited until I thought of a new way to use it or I just didn't care if it sucked.
I know in my CSM I am focusing on Nurgle and I am going to use some or all of the below units.
- Land Raiders (Yes, I want two!)
- I want to start using an old Mark I land raider I have on my shelf
- Plague Marines
- Demon Prince (Nurgle, of course)