Having seen some excellent results on bird photos recently posted using flash and Better Beamers, I'd like to give them a try. I'd be grateful for advice based on my wanting flash for bird photos (using a 5DIII and 300mm f/2.8 plus extenders).
580 or 600 flashes (and there's no advantage to the 600, since the higher guide number is only due to the greater head zoom, and with a Better Beamer you'll manually set your zoom between 35-80mm). I recommend the more powerful flash because even though you'd dial back the power with a Beamer (-2 FEC if using E-TTL), with birds you'll want a fast shutter, and faster than Xsync means using the power-robbing HSS.
Also, it helps to get the flash off-axis to avoid steel-eye (red-eye effect). I use a RRS B91-QR bracket (attached to the lens foot) and an FA-QREX2 extender, along with an OC-E3 cord to get the flash elevated.
I've only been using the Better Beamer for less than 2 months (I've heard of it through neuroanatomist in CR) and I'm really impressed with what they can do. When I'm shooting with my 5D MK III + Sigma 150-500 OS, I use 600EX-RT speedlite ... I highly recommend it, it is just awesome. When I shoot with Nikon D7000+300 VR (which I carry everyday due to convenience of not having to change the lens) I use Yongnuo YN568EX - its a decent speedlite for under $200 and does a pretty good job.
Here are a couple of shots:
1st one using Canon 600 EX-RT + better beamer on 5D MK III & Sigma 150-500 OS at 500mm
2nd one using YongnuoYN568EX + better beamer on Nikon D7000 & 18-300 VR at 300mm