In this specific situation where 600C is already well designed horizontal center (at least James likes its smaller brother - 500C) the difference isn't that big to be concerned but in many cases, vertical orientation beats horizontal.
I recommend reading these two articles:
This article discusses performance tradeoffs of vertically vs horizontally mounted drivers in center channel speakers. We show how off-axis lobing can negatively affect performance of horizontal MTMs.
www.audioholics.com
This article contains an in depth discussion of the trade-offs of different center channel designs (MTM vs WTMW, etc) and their real world applications and limitations.
www.audioholics.com
As Pogre correctly said, a horizontal center would be useful if you ever find yourself in a none-AT screen situation.
As for the next upgrade, you may want to consider a subwoofer upgrade. don't get me wrong, dual X12 are force to be reckoned with, but they are a bit weaker in lower bass (below 40hz). It really depends on your typical usage and how much of a bassaholics are you