I've used both VIA K8T890 and nForce4 (SLI) based motherboards. If they both work properly, you propably won't notice the difference. I think the most important thing with motherboards is the stability.
My upgrade 4 months ago was ASUS A8V-E Deluxe (VIA K8T890), and I had very hard time to get it work properly: I had problems with system stability, with my SATA drives and with my IDE-optical drives. It might be more because of A8V-E (No T1 memory timings and stability was horrible) and its bad design, but after fighting with it for 3 weeks I got replaced with ASUS A8N-SLI Deluxe. My system works solidly now (has been for over 3 months), no problems whatsoever.
I think other manufacturers K8T890 boards work better (like Abit AX8), I've heard good things about them, but the ASUS K8T890-based board left me with a very bad memory/feeling about VIA-chipsets.
I would recommend getting the nForce4 -based motherboard. For me it has been more solid and although it has lot of bells of whistles It's still the better chipset at the moment. in my opinion.