Both have their strengths, so it depends on your applications and print quality requirements. RISO digital duplicators are simple enough to operate that a highly skilled operator is not required. They are also simpler to change from one spot color to another (20 seconds to switch color drums) and do not require any color wash. There is no make ready - the first print is sellable. At run length of 1,000, the total cost (consumables and service click) for a two-color job is less than $8.00 and the total production time is about eight minutes. RISO digital duplicators can also print spot color envelopes all day long at up to 180 per minute.
With all that said, digital duplicators cannot print above a 100 lpi screen or on glossy media. Print quality is improved to 600x600 dpi, but most people think offset quality is better, especially for halftones. Registration is +/- 1 mm.
I know a lot of printers who have bought a new RISO digital duplicator but have kept their offset presses for higher quality work.