Help choosing a control board

Hey guys! So I've decided to build a DIY Taz, and I'm trying to decide on a control board so I can get a complete BOM together.

I've been looking at mostly the Ramps and Rambo boards, but I guess I'm open to really anything. Here's what I want out of it:

• Support for 6 Steppers (4 for X, Y, and Z, 2 for dual extruders)
• Support for large heated bed (12"x12") through single power supply
• Works with 24V/17A/400W power supply (I think this is necessary?)
• Supports LCD and running from SD (so I can add that later)
• Is common/popular so I can find documentation and support

Based on my research, I think both the Ramps and Rambo boards will do that, but I'm a little confused about how each work.

Does the Rambo version matter? 1.2A boards are cheap on Ebay, but Ultimachine has the 1.2H for twice as much. What's the difference?

Also, I was looking at this heatbed: [www.ebay.com]

And they say it requires a separate power supply. Why? Is it simply impossible to use a single power supply with a heatbed that large?

For Ramps, could someone recommend a good kit or set of parts? Either separately or together as a kit, it doesn't really matter. Cost is the key. I don't need the LCD right now, so just the Arduino, the Ramps board, whatever stepper drivers are required, and anything else I may need (for instance, to control the heat bed).

Thanks in advance!