Even when its used for data - one channel, also called time slot is used for control/framing, it can be used to signal receiver end, start or end of frames, which can be used for detecting any lost bits etc. for CRC purposes. So in short even though e1 is said to have 2048kbps and t1 1544 kbps you will not get full bandwidths and will have usable bandwidth approximately close to as mentioned in the above posts after leaving out d-channel and framing over head.
To add you can use full e1 bandwidth only if your telco is selling clear channel e1,which they normaly donot sell - mostly you will be getting channelised E1.
Your provider will only provide you either E1 or T1 - T1 is used in US, Canada, Japan and E1 is pretty much used in rest of the world.
CAS, CCS or RBS are the signalling methods. You can think of these as protocols that run on top of T1/E1. RBS is the oldest one more closely replaced by CAS. CCS is the most recent one - PRI uses CCS. Without going much into signalling you should check with your provider for the options, what are they offering.
Its already explained above - physically both E1 and T1 are different - and offer different bandwidths. Both are different standards followed by different countries. You will not have choice in choosing T1 or E1, your provider will offer the circuit based on the standard your country is following either E1 or T1.
