I used both g2 and g1 quite a lot.
I find that g1 uses a lot of hard disk access, so more ram might be required, eaccerlator won't do much to speed it up.
g2 seem to do a lot more database inquires, but that is obvious as g1 doesn't even use database. As far as speed wise. Say if you have a Pentium 4 2.4 with 1 gig of ram. Before 2000 images, G1 is faster, after around 2000 - 3000 images, g2 become faster.
g2 also speed up -a lot- after installing eaccerlator, somewhere about 300 - 400% on my count. g1 hardly speed up at all.
So it depends on what you need and what you have. If you have a hosting enviroment where you can have root access, or get someone who have root access to install eaccerlator. ( i uses 0.9.3) G2 is better in almost every way except some of the modules that are still in development. Like a comment block, e-mail notification, or voting system. But some are in the work and beta/preview version are available on most thing.
If you have a smaller gallery with less then say 1000 images, stick with g1. It will have similar speed with coppermine from my experience, but much more flexible and feature rich.