My recommendation is to go straight to the ds-125. The odds are that you will want to upgrade reasonably quickly so why buy twice. The ds-125 provides greater coverage for w/a shots and will be more forgiving on strobe placement when you first start out. As far as basic vs. complex, you will probably notice little difference in the two as far as complexity of use.
One other thing that I forgot to mention (and to me is a big selling point of the ds-125) is the light which can subsitute for your primary light on night dives. The ds-125 has one, the ds-50 doesn't.
My history is that I thought, well, I don't want to spend all the money for a ds-125 and I want to see if I like taking pictures underwater. It didn't take me long to want a ds-125 so I bought one. I figured, I could use the ds-125 and ds-50 together for awhile but wasn't completely satisifed with the results (some shadows due to different strengths). Which is why the second ds-125 was delivered a couple of days ago.
For me, it isn't a complete loss. My wife wants to have a small compact set-up to try underwater so I am setting up an u/w system for her and will use the ds-50 on her camera.
Finally, one last comment, I have had such great luck with Ikelite's service that I am a big fan. Very few companies, in my experience, are so interested in customer service.