This is a frequently asked question. So rather than write the answer over and over again, I figured I'd just answer it once in this blog post.
Question: I have X Gold and Y Command Points and Heroes A, B, C, D, E, F, ... Who should I buy?
General Answer: It depends on your answer to the question: "What are you trying to do?"
"Not trying to accomplish anything in particular."
In that case, then it doesn't matter who you recruit next. Recruit which ever hero strikes your fancy. Have fun. Pick your favorite hero. Alternatively, recruit nobody. Save up your Gold and Command Points until a hero that you really really want comes along.
"I would like to do better in PvP."
In that case you should buy lockboxes until you can recruit Juggernaut and spend Command Points on Quicksilver and his Blue Suit alt. Juggernaut + Blue Suit Quick Silver (Generalist) is the top PvP team. Juggernaut + Swashbuckler Nightcrawler is another formidable PvP team often used by top PvP players. After you have recruited Juggernaut and Quicksilver, then start recruiting Blasters to boost your Hero Bonus.
"I enjoy playing PvE."
In that case, you should save up your Gold and Command Points and wait until the next Special Ops comes along and then recruit the heroes needed for the Epic Boss. This will allow you to progress through the Spec Ops tasks to recruit the Spec Ops hero. In general it will take you upwards of 400 Command Points to get all the heroes needed for the Epic Boss deploys if you don't have any of them. If you haven't already recruited Storm, you might want to do so in order to farm Chapter 4 Mission for more Command Points.
"That's nice. You've been no help whatsoever."
Yeah, sorry. Everyone has their own favorite heroes, teams, and strategies. If you browse around on this wiki you will find lots of advice about who you should recruit. In the end you should chart your own path. Recruit heroes as needed to easily win fights, PvE or PvP. I find that I often put together teams that include Psylocke, Juggernaut, Havok, Black Panther, and Magik/Ghost Rider.
You will need various heroes to unlock Epic Bosses and Premium Missions. Again, if you browse this wiki you will discover which heroes are required to unlock what.
Personally, I don't bother with alternate uniforms, because I'm just interested in recruiting all the heroes and I rarely have enough Command Points to spare to recruit alternate uniforms. However, I have recruited Iron Man Mk-42 (makes Iron Man somewhat useful again), Iron Patriot (I thought it would make War Hammer a force in PvP, but turns out not to be as potent as I thought) and Future Foundation Invisible Woman (got the Blaster uniform because the suit looks great and I wanted to move my hero bonus toward blaster). Those are my choices, you need to make your own.