Naturally, the performance is gonna be better on any newer model compared to an older model. Faster CPU, more/faster RAM, bigger/faster hard drives, better WiFi, etc.
SL is supposedly lacking in build quality (plastics, fit-n-finish, ruggedness) compared to the T-series, although still feels quite durable. It is also missing some traditional Thinkpad features, such as docking station support, Thinklight, removable optical drive, etc, but also has quite a few other features which an old model such as T43 does not have, such as Firewire, more USB ports, built-in camera (on some models), HDMI output.
On the other hand, T43 has a PC Card slot, which SL doesn't have (they have only ExpressCard), and parallel/S-video ports, which are no longer present on any modern Thinkpads.
In your particular case, T43 might have an additional advantage of being compatible with all your T40 accessories, if you have any (such as power adapters, docking stations, batteries).
Current: T410 2537-R46, T60 1952-F76, T60 2007-QPG, T42 2373-F7G, X61 7673-V2V
Collectibles: X32 w/ IPS Screen, A31p w/ Ultrabay Numpad
Past: Z61t 9440-A23, T60 2623-D3U, X32 2884-M5U