The "Other Istanbuls"
Most everybody interested in Handmade Turkish Cymbals has heard the story of Mehmet Tamdeger and Agop Tomurcuk learning the craft at Mikael Zilcan's factory from early on, and establishing Istanbul Cymbals in the early eighties to produce the now coveted green logo'd "pre-split" Istanbuls. After Agop's death in 1996, the company divided into Istanbul Mehmet, still run by Tamdeger, and Istanbul Agop, headed by his sons.
There is much discussion on cymbal forums like cymbalholics.com and others about the presumed tonal differences between the two brands (see Links). The Mehmets have been described as "smoother", the Agops as "trashier" and vice versa. Personally, I find both brands produce outstanding cymbals, and the "pre-split" myth is mostly that.
Clearly, Istanbul Agop has done a much better job at promoting and distributing their cymbals up to now, signing up less obscure, more internationally known endorsers etc. (maybe a "generational" advantage), but all that PR has come at a price that gets passed on to the consumer (Mehmets have generally retailed for at least 10+% less).
As a frugal kind of consumer, I'd rather play lesser known and endorsed cymbals at a better price. Call me smart.

Buying Cymbals Online: My $.002
To some people, it seems like the worst possible idea. Others have purchased their entire set-up on the web. My own experience as an online customer of cymbals and drums has nearly always been positive. As a matter of fact, the only cymbal I ever disliked enough to return it, was bought at a brick-and-mortar store ( a Meinl crash, btw., and I am German).
Granted, sound files have their limitations, and the actual cymbal you are buying may not sound "exactly" like you expected, but sometimes that's even true for the "soundfile" in your head, uploaded in a specific actual room (store). "Great sounding" cymbals should sound "great" in almost any setting, imo.
In the rare instance you do want to return a cymbal, you only pay for the actual shipping costs. Considering the often substantial online savings, I believe it is usually a gamble worth taking.
The new Mehmet Constantinople series (2010)