It all tends to go hand in hand for me. I'm a thrifty sort though; while I sketch a lot of ideas for 'scapes when it comes to actually setting one up I try to keep purchases to a minimum. So I've always got a mental list in my head of what materials I have on hand or can switch out from other tanks.
Helped along by the fact that 90% of my set-ups are 3g and under. Easy to plant them up with just trimmings from my main tanks, hard to justify paying shipping for a particular plant when I only need one. And, honestly, I'm a tinkerer at heart. I don't think I'll ever be one of those who draw up an exact plan, follow it to the t and end up with a tank that looks EXACTLY like my original sketched plan.