Thank you all for your advice and suggestions. You've given me some ideas and perspectives that I hadn't thought about and that's exactly what I wanted.
Neuro, thank you for the link to Aperture Inspector. You or someone else had mentioned it before, but I couldn't find it in the App Store. Now I know why! I watched the demo and I'm going to get it.
I plan to keep the T1i, at least until I get a replacement for it, although that won't be right away. I really like the smaller size and weight for times when I don't want to take the heavier 6D and L lenses with me. It's not much bigger than the new SL1, although clearly the T1i has older technology.
Aroo, you asked why I still have the 28-105. It was the lens I bought with my Canon Elan 7 in 2001 and I kept it when I sold the Elan 7. I thought it might be a better lens than the EF-S 18-55, but after using the comparison tool in The Digital Picture, it looks like the 18-55 is sharper, although the 28-105 is a faster lens. Am I reading the chart info in TDP correctly?
Wsmith96 suggested I keep the 18-55, which I could bundle with the T1i body when I want to sell it. Assuming I've correctly understood the information from TDP, perhaps I should sell the 28-105 and keep the 18-55?
Any additional thoughts will be appreciated.