I've given this issue some thought, but ultimately it comes down tho two things:
1 - I'm building this layout in 2009 and to me it makes sense to use the latest technology. There's no cost barrier for me in terms of pre-existing stock, and I haven't accumulated that much, and I'll convert locos as I go. Most new products come DCC-ready or have well-documented conversion approaches.
2 - Operationally, Beattock was all about bankers. I need a way of controlling two locos at once and while this could be done in DC it's not simple or reliable. In DCC it should just be a case of creating a 'consist' ( which can be preset or done as needed) and then the two locos will operate together. I guess there may be some issues of speed differences, but I'll cross those as and when. DCC also makes double heading easy, and there was a lot of that going on a Beattock too.