I dreamt last night that I kept missing calls from Benedict Cumberbatch. It's a sign, I tell you! A SIGN! *waves hands and rolls eyes wildly*
Personally, I think you couldn't buy one without the other. I am in no way, shape or form a completionist, but I would have to buy a Watson if I were to buy a Holmes. It's an essential accessory.
I am happy to wait a little longer as I want to see how BC's quality holds up. I am not at all impressed with 10's tailoring and need to see if that side of things improves. They could do with getting someone else to do the paintwork, too.
And a side note. I was at MCM Expo in October and saw 11 on the Forbidden Planet stand. A friend told me that they were selling him there for £100 (still full price on their website). I can't confirm this, as I had no reason to ask (I already have an 11) but, if this is the case, I won't buy these as soon as they are released, but will wait and hound the Forbidden Planet stand every time I see it at a con. £100 is just about the price I am happy to pay for this quality figure, without kvetching endlessly about dribbly paintwork and buttons and braces falling off.