You need to physically try them out. I have 5D, 40D, XSi. The XSi is a great camera; it feels very small after using the 5D. I keep it for a lightweight always in the car camera, love it. That said, I notice I always use the 5D or 40D when choosing a camera for the day. If you are trading up, it will be 200% better than what you have. Both the 40D nd XSi will give you the photos you are looking for.
If this is something you re going to keep a long time, you might handle the 40D as it is a lot more substantial. My husband hates the XSi because he feels it is too small for his hands. You can get a 40D for really good price right now; your post sounds like you might be happier with it.
I have both an xti and 40d. Features ... not an issue. Image quality ... not an issue. It's really all about the handling. And, quite frankly, that's not that big an issue if you're using shorter lenses. I've been shooting a lot of sports in the last year, using longer and heavier lenses. The 40d handling is substantially better than the xti with a 70-200 2.8 or a 100-400 attached. However, when we take a day trip or vacation, the 40d stays home in favor of the smaller, lighter xti (and the heaviest lens is a 70-200 4.0) I'm 6'2", so I do like the feel of a larger camera body, but with the shorter, lighter lenses it really isn't that big of a deal.
Go to a store that carries them both (or similar) and handle them both. You may find you like the lighter weight of the xsi or you may find you like the solid feel of the 40d. In my opinion, the handling is the most significant factor here and there's an arguement for either camera.
Got both and if I had to chose I would go for the 40D with the tammy with no doubt.
ISO 3200
9 Cross point AF
6 Shots per second
Ergonomics
Compact Flash cards
Better balance with bigger lens
The control wheel
More Durable shutter
Better weather sealing
The actual price is a bargain
These would be my points in to chose the 40D over the 450D, now, if you want a few more pixels for landscape and birds...maybe the 450 would be a better option
Check the Tamron out carefully, some are not sharp right across the frame (yours looks fine to me though from what we can see here), when you do get a good one they are spectacular
Fortunately, they allowed us to check the lens in the store (on my friends camera) and shoot some shots to see if it's sharp. It's also from Japan, and from what I've read, the QC there is better than the chinese version. All in all I'm very happy with it
Tweak I own a 40D, my wife has an XTi and my son an XS. I also own a 100 macro for work and fun, a 10-22 which I love,( we have the cheap kit lens that came with my wife's that I wouldn't bother to use) and we have a lens that the local shop owner recommended to me that I have found to be a very good all around lens. It is the Tamron 28-300 3.5-5.6 Aspherical AF Macro. My son has the Tamron 28-200. Depending on your uses and needs you may want to consider this 28-300 option as it is likely within your budget + or -. As far as the body goes, I have large hands and find the balance of the 40D more satisfactory, but I have used the XTi without feeling unduly bothered. Ultimately until you get into the very expensive full sensor bodies, the quality of your glass will have a greater impact on your images than the difference between the 40D and the X series. Having said that, I would still opt for the 40D if you are not in a position to consider upgrading for a long while. Just my opinion. m
