If you're on a tight budget, consider this. Get WD now and use for your editing, as you grow then buy better drives for editing and relegate this WD to backup. You have thought about and included backup in the budget, right
The overall length of your projects isn't so much the problem, it is more about the source footage. AVCHD H.264 is very disk intensive, for example. So depending on how much color correcting, adding Effects, layering generators, etc. you do then the slower the drive becomes an issue. That said, when you import your clips, if you transcode to Proxy or Optimized (ProRes) then disk I/O will effectively improve. Since you're getting the good modern rMBP and talking about "short" Projects, the time hit probably wouldn't be too bad.
If you're willing to scout around I would imagine you might find a 7200 RPM spindle HD that's just USB3 for a better price. But if you need to just get going, I contend the 5400 will get you started. That dual interface OWC drive is just not the right fit for you
The 1TB OWC On-The-Go drive is great. They are 7200rpm drives, and USB 3.0. I use a couple and love them. The 2TB is 5200rpm, due to heat issues, so stay away from that one. Most bus powered drives larger than 1tB are going to be 5400rpm due to power consumption and heat issues.
I'll say from experience, even SD short projects on any 5400rpm drive will eventually drive you crazy. Spend the extra few bucks.
Personally I now stay away from WD portable drives.
WD has decided to add password required protection into drive firmware, big PITA.
Also WD drives are not real reliable over long haul. HGST (Hitachi) drives are proving to be most reliable, who knew.
I recommend getting a USB3 dock that has USAP enabled and buy bare drives to stick in and swap out.
Check out the StarTech docks with USAP.
The dual and quad bay docks can be configured as RAID 0/1 via Disk Utility increasing speed if needed. They also support cloning. Not bus powered however.
Other USB3 docks/enclosures out there but not all support UASP.