Jon has the more memorable voice, and his control and stamina in his (albeit pretty short-lived) prime was extremely impressive. But I can understand why he might be a bit grating for some people, and Richie is probably more universally palatable in his sound, though it is less distinctive. All things considered, prime voice Jon all the way, but Richie has held up over the years better and is the better singer thesedays.
His voice started changing in 1995 (due to becoming a habitual smoker the year prior), but he still remained a great singer (live and studio) up to 2000. His voice by the Crush era was thinner and a bit more nasal compared to a couple years prior - but he still sounded decent enough for the next couple of years. His voice took a big downturn after the American 2003 Bounce tour leg (the spring months) and that's when he really started to struggle (especially live). He lost his technique and has sounded pretty awful since then (albeit with some minor recoveries from time to time).