Personally I don't think that you can choose a laptop on the processor alone, there are far more important considerations than just the processor performance. That said if you look at the specifications for the two you have mentioned you will find that they both have the same power consumption (35Watts), the same L2 Cache (1Mb) and the T3400 is rated at 2.16GHz while the T3000's speed is slightly less at 1.90GHz.
click here is the Toshiba model comparison site which might give you an idea of what variations you can get and help you decide which is the best laptop for you. You can use the comparison to build up the spec that you want and then search around for other makes with that spec if you want. Don't forget to compare warranties and after sales support.