I'm not looking for high-end IQ but for, instead, convenience and reliability as I try to learn about composition and exposure. I shoot just for myself, don't exhibit, don't do posters, etc.
Owing to LBA and a credit card
, I have:
DA 40. Superb lens .... but now that I have more zooms it doesn't get on my camera much unless I'm doing something very specific. I'm glad this was Criagslist at half the MSRP. It may go back there to help fund a K2000.
Sigma 18-50 2.8 (older non-macro, closeout price) Good all-around, particularly indoors. This is good for shots of the twins, age seven months. Crisp images, very helpful DOF from the constant 2.8, and fast focus in low light compared to the ....
Sigma 17-70 Good all-around, house and garden lens. I like and use the extra 20mm but it has more difficulty in focusing in low light owing, I imagine, to the aperture. Good walkabout lens except that I somehow end up screwing on the ....
Tamron 18-250. Not very fast but this one seems stuck to my camera if I'm walking about campus, at a festival or fair, at an airfield for a fly-in or air show, etc. Acceptable images and it gives me the ability to get the shot instead of mucking about with lens changes. It's the ultimate holiday/vacation lens for me and a lot of others.
For an outdoor event or ramble, I usually take just the 18-250 and the same Sigma 10-20 you have. I'm covered for almost anything, don't spend much time choosing and changing lenses, and can easily carry a day's gear in a photo vest.
I suggest the 18-250 as it's in your price range and allows you to see which focal lengths you really use. If the long end doesn't turn out to have much value, you can sell it on and choose the right lens for you from your own actual experience rather than from suggestions like the one I just made.