grunzster:I'm still trying to decide between 100s or 120s. I'll use them as singles at first, but will eventually double them. I'm a little guy, so I think the 120s may be a little big for me to handle as doubles.
On the other hand, if I'm going to keep up with Martin, I may need to go for the 130s!
I have 120's that I was using for singles, When I went to doubles, I was worried about the weight, so I purchased 100's. Now after a little over a year dive the 100's I am doubling my 120's. One, there isn't enough gas for the deeper dives which allow me to keep a proper reserve. Two, When sitting down getting geared up, they are a little short and I find myself having to lean back while getting ready. This is a little tough when I am ready to get up, I always need up. Now I am not that tall, 5'9".
Take your BC in the shop and put both tanks in, see what you think.
Good Luck