Sup PikachuGuy, it's nice to see you coming here to ask questions!
Choosing between Alakazam and Mewtwo really depends on your preference. One of the main things you should think about is what tiers they are in. These two pokemon are situated in two tiers, Ubers, and RU(although Alakazam is likely to be bumped up to UU if not OU.) For a complete list of the tiers, go here: http://www.smogon.com/bw/tiers/
Now onto the real question: Which one will you pick? Well, I myself prefer Alakazam in a standard match. This is because I'm not much of a competitive Uber player and play more standard BW OU. However, if there are no restrictions, go with Mewtwo. This is because Mewtwo has significantly better stats, a wider movepool to abuse, and is generally one of the deadliest sweepers- even in ubers, at least to my knowledge.
-Lead = Forretress (explained above)
-Crazy Uber *****y Destroyaaah = Swellow - Takes out at least 2/3 team members, and sometimes wins the match for me. She survives priority of any kind like a champ, normally not taking much, and OHKOs everything in sight that's not steel/rock. Also pretty nice for revenge kills, as she can get toxic orb working right away. (Brave Bird + Facade + Quick Attack + Pursuit)
-Fast Set up Sweeper = Gyarados - When I am able to set him up with one/two DDs, he's beast. Only works when I predict correctly - which is 70/30
-Scarfing Hard Hitting Beast = Hydreigon. Basicly an all-out fast+strong combo, even though it normally only stays in for a couple of turns max., since it has a ton of counters. He's pretty awesome though. (Draco Meteor + Fire Blast + Dark Pulse + Stone Edge)
-Slow Set Up Bulky Sweeper = Eelektross (physical, with Wild Volt and Thrash... I love normal wrecking attacks, and with the high attack it gets, it's plummels through a team with at least a coil or two. Being slow is his main flaw, but he can take most hits)
-Support/Wall = Blissey (Heal Bell + Wish + Flamethrower/Hyper Beam (she's hardcore) + Thunder Wave - for those who aren't affected by Toxic Spikes)
I'll post it with more detail a bit later, since I'm not at home and I don't have their team status (also not sure about some of the things mentioned, like Hydreigon in general, since I only remember having a scarf and the only scarfs in the box that make sense here are him and Darmanitan, but I also remember having 3 physical attackers and 1 special). But I was thinking about getting a Milotic instead of Gyarados, since the 4x weakness to electric bugs me incase one shows up (even though grass being neutral is useful), and because setting up DDs the way he does takes a ton of health away, and sometimes kills, being easy bait for the ever present Scizor (almost OHKO'd after SR damage). Milotic is also bulky as all hell, even without rain present.
What also worries me is me being mainly physical (like I said, attackers = 3 physical and 1 special).
Please no suggestions of Ferrothorn.
I haven't played with this team much though, since before I was trying out Infernape one more time. OHKO's Gliscor after one Overheat, which is awesome.
Thank you for your time, I guess x)
Thank you for coming by and to both of you, have a nice day!