The 170 series also measures capacitance. If you look at the
comparison page I linked to earlier, you can see that the 180 series differs from the 170 series in terms of range for certain measurements and basic DC accuracy. Also, the 180 series does true RMS readings for AC+DC, while the 170 series only does it for AC.
The 189 is working out great for me so far. I didn't really look at the 170 series before getting the 189. I bought my 189 from Fry's, a local electronics retailer that everbody hates, but it's the only game in town, so we shop there anyways... I paid around $410. I don't think I'll ever use the logging feature, but the 187 and 189 were almost the same price.
If I were still a student, I probably would have done a little more research and considered my needs more carefully before buying a new DMM. I also probably would have looked on eBay, instead of going for the instant gratification of buying it at Fry's. As it is, I work too much and have limited time for personal projects, so I bought the 189 because I knew I wouldn't have to deal with DMM-related issues.