random... nearly always. I get way too bored if I try to play a few games in a row with just one race. Generally I only pick if I random the first 4 or 5 games and it happens to never pick one of them, or if it gives me the same one 2-3 times in a row and I really don't want it again, I'll pick one of the other two randomly.
I'm not one of those people who memorizes their starts or where they place what on the map.... almost every time I play it's a little bit different, and I'm fast enough that I can hang with decent people (I did beat the campaign all the way through, and some of those last zerg levels in the expansion were really nasty... so I am competent), but not enough into it to really practice getting better at things.
SC and tiberium sun don't have a lot in common, tbh. SC runs at a much, much faster pace, and the three races are varied far more than the two factions of c&c were. Like... in all the c&c games that I can think of, each side had basically a one-to-one correspondence of similar troops. You have footmen who are good at killing soldiers, but bad at tanks, and footmen that are mediocre/bad at killing soldiers, but good at vehicles. Then vehicles that are good at killing soldiers, but bad with other vehicles, and vehicles that are mediocre at killing soldiers, but good against other vehicles.
By comparison, SC is very... uneven, in it's power and cost per unit. The first tier of one race's(protoss) "grunt" soldiers can kill 2-3 of any other race's. But another race (terran) gets the benefit of ranged grunt-men, and the ability to build bunkers really early in the game, which they can stock with low-level soldiers to protect them while they shoot. The last race has extremely weak guys to start, but they come out 2 at a time, and you can build 3 pairs at a time, as opposed to the other races, which can only build one at a time. In addition, all these troops cost different amounts of money. Then shortly after, there's a "second tier" of ground units, which all interact with the first/second tiers of other races in different ways.
That sort of layered-balance is scattered throughout the game. That race with the beastly-strong grunt-men have hit-points that are split up, half shield and half life... so if you fight with them briefly, their shields eventually recharge slowly.... though any damage done to their "life" points is permanent. By contrast, the race with the cheap troops that come in pairs are supposed to be biologically advanced, so they regenerate constantly (though it's extremely slow). The last race (with ranged guys) get no natural life regen, but can build medics (which heal soldiers extremely quickly... almost instantly) until they run out of energy (which constantly recharges), and repair their buildings (so those bunkers you stock full of soldiers can be repaired in-battle).
By comparison, c&c is extremely straightforward. You build guys... but one side's troops are roughly the same as the other's, and there's not as much "ok, now what can I do to take care of *those* kinds of troops". You just sort of need to harvest a lot, build more than the other guy, and go take him out. At least.... that's the impression I got while playing it. Again, though, I only did do the campaign.
