I would say its a whole package, you can have the best legs around but if the head is hard to use, sticky or sloppy then the system isn't very good overall. If your only looking at purchasing one expensive part at a time I would say go with a good head since most tripods do alright if the legs are kept short. Cheap tripods really start to show there downfall when the legs get extended to standing height.
I spent most of my allocated money on a nice set of binos, so I'm looking at a cheaper tripod to hold me over until I can get something nicer. I was originally thinking a Slik sprint pro ez for $100, but i came across a deal for a manfrotto mvh500ah head and Benro a2970f legs. Both retail over my budget but together someone is selling for $150, which is as high as I want to go. it also weighs 6.5 lbs, which is probably too heavy for the mountains. It got me thinking if I should just buy the nicer head or legs and get a cheaper one of whichever I don't get from this combo.
Get the good legs (can get the promaster for 250), then scrap and save the extra $75 for the Benro S2 and you'll have a great system. The legs are the biggest chunk so do that first. Makes it easier to justify the little bit extra to have the setup done right the first time...
I'd save up for the Promaster legs and 6160 head. The head can be replaced later if you choose, but it's pretty darn good for the money. I used mine with no complaints until I sprung for the Outdoorsmans head later. It's going to be double your budget but I'd rather buy that set-up once than spend $150 now and want to upgrade it next year. If you need to get something cheaper for this year maybe a Vortex High Country with their ball head that can be a back-up or sold next year. Personally I'd try to save up and get a good one from the start.