Here are the differences in design that I know of:
1. Zeagle has a slightly larger heat exchanger in the second stage and the second stage appears a little smaller.
2. The HP design of the Zeagle is from an engineering point a view possibly longer lasting. ( time will tell) The reason is hard to explain without a picture but here goes.
http://www.zeagle.com/reg2001_design.htm
The previous Apeks design ( and most others) features a seat arrangement very similiar to the LP seat in your regulator except harder materials. the HP seat assembly seals against the raised metal cone and a ring shaped impression develops quickly. Eventually the ring will become too deep or irregular and the seat will fail or "creep"
The new seat design features a metal cone which locates into a cylinder of plastic seat material which should allow more accurate positioning over multiple cycles and in theory offer a longer life. ( it looked good when I was at the reg. clinic)
3. Zeagle also has a full lifetime warrenty providing you get it serviced yearly. ( means your parts are free you just pay for labour).
I am still diving DS4/TX50's but am seriously looking at buying two of the flat head sixes to try out as they have spent a lot of time working on these reg designs and I liked the people I met who were behind them.
On a side note, Jim Fox who is one of there lead reg guys worked at Sherwood for many years before they pulled the rug out from under most of their staff and handed back all their distribution to the Liberty group ( genseis/akona).
Zeagle recognized an opportunity and hired him asap . Jim is a smart cookie and while Sherwood are not known as high performance he has brought one of sherwoods biggest mantras to zeagle which is reliability and ease of service.
I think that zeagle is gaining more market share and once they get out of the shadow of Apeks they will come into their own.