What are the handling advantages/disadvantages in riding wider rims? i.e. Mavic 717 vs 321 or DT 4.2 vs 5.1

Have been looking at Hope Pro II hubs and DT 340 hubs. The DT Hubs seam to have a good reputation but how easy are they to rebuild, get spares for and do I need any special tools? The Hopes are easy and I have the tools.

The wheels are for DMR Trailstar LT, its a lightweight build, but will be ridden hard up and down the black run at Glentress and asked to play at the freeride park from time to time.

Have been riding a big forked Giant NRS on Pro built Hope XC hubs with Mavic 717 rims and they have been perfect. But suspect they're on the limit now and wonder if they will struggle with the DMR hardtail.

Wider rims generally mean wider tyres will sit more comfortably, and you should be able to run them at lower pressures. They should also be a bit stronger, but they will be heavier, and roll a bit slower.
As for hubs, you really wont go far wrong with Hope's. Light, (relatively)cheap, and a great reputation.

You can get wide hubs that are pretty lighy (the DT 5.1 are a good example) that would be grat for a tough hardtail.