I've got one Einstein, three 600EX-RTs and an ST-E3-RT, plus a PW MiniTT1, two FlexTT5s and a PowerMC2 for the Einstein.
PW's are pretty expensive, but they've worked well for me. I do like the rapid control over output you get with the AC3. But the Canon RT system is easy to use, and very reliable.
If I want to use a big modifier (48" octabox), I use the Einstein. I have Lastolite boxes for the Speedlites (two 24" Ezyboxes and a 12x48" stripbox).
There are a couple of ways to blend the systems. The Einstein has a built in optical slave trigger, so as long as you're using the Speedlites in manual (no E-TTL pre flash), that works. It's also possible to put the ST-E3 atop the MiniTT1, triggering the Einstein's PowerMC2 with PW RF and the 600s with Canon RF.