Matt, Welcome to the group, and don't worry, heated (but friendly!) debate is very much the order of the day here!
A few years ago, the biggest reason for using EM as opposed to OO was the improvement in appearance. EM layouts simply looked right, and OO, especially code 100 track based ones just looked too much like a train set by comparison. As Ian has mentioned, many locos were easy to convert to EM, and since much of your stock was going to be kit built anyway, it was just as easy to build them to the wider guage as it was to build them using 16.5mm.
Now, the story is different. A combination of SMP (or C&L) track and kit built points, and a much wider range of stock for most railways (I know we still whinge about our favourite loco's, but think about what it was like in 1990! No Bachmann and the Hornby/Triang Princess was the height of detail!) are now available with a much higher level of detail. You put one of todays locos next to one of the best kit built models of a few years ago and you would be hard put to tell them apart.
In short, OO is now a much stronger competitor to EM in terms of appearance, and wins when it comes to getting things running ASAP.