I've not used the HS10 but comments I've read from folks who have have upgraded or at least compared the two have been mixed. Mostly I think people recon a HS10 with CR40(?) filter, calibrated well is close to a HS20 (although they don't tend to give the HS20 the benefit of the perfecting ).
For out of the box most seem very happy with the upgrade.
Personally I'm very impressed with my HS20 but it's my first PJ so no surprise there. All of the demos I'd had were with DLP PJs and I do notice a drop in contrast and higher black levels. But what I don't see is lots of rainbows and it certainly quieter then models I'd demo'd.
Out of the three PJs on my short list the HS20 was the only one I could actually get ahold of for months, the cheapest (by £1000), the best I/O, the only powered Zoom/focus, the only true XGA capable for PC use and the only rainbow free one (the others were DLP HD2/HD2+ based).
I'd reconmend looking at some good DLP HD2/HD2+ machines and see if you're happy with the rainbows, I think you might not get enough wow out of a HS10->HS20 upgrade. I plan to look again at DLPs in a year or so when hopefully availablity/technology has improved enough to be able to buy one. In the mean time I'm happy with one of the best LCD based PJ on the market, a saving of £1000 and hopefully on Tues a nice shiny electric screen so I can finally pull down my bedsheet.