You can have a sequence running on one patch and play on another, yes. You can have a sequenced synth and a playable synth in the SAME patch if you want! The possibilities for sequencing are vast with the Nord Modulars.
Off the top of my head, some key differences between the G1 and the G2 are:
G2 has MIDI modules, G1 does not.
G2 has LCD scribble strips above the control panel knobs, G1 does not.
G2 keyboard (and G2x) has a very elegant physical user interface that make it a joy to use.
G2 has more memory for DSP effects like delay and reverb, G1 does not.
The general consensus is that the G1 sounds a bit more aggressive than the G2.
A huge plus for the G2 for me is that the demo editor lets you make (and hear) sounds without needing a G2. I love this because I can make patches at work on my break or whatever and then perfect them when I get home. If you want to get an idea of what the G2 would be like, go here and get the demo: http://www.clavia.se/products/nordmodular/demo.htm. Then, go here and download some patches from the sequencer section and see what can be done (some won't work with the demo version, but many will) http://electro-music.com/forum/forum-44.html
If the sequencers are something you think you'd be using often, I'd strongly suggest the G2 keyboard. When you map a sequencer to the control panel, the knobs are mapped to each sequencer step, and a chaser-light runs along the bottom when the sequence is playing. It's very slick. Plus the G2 has MIDI modules (not available on the original NM) so you can sequence whatever you want with it. I've had mine for a while so I don't really know what they go for today.