Hi MP5 RAS,
I own both a TM M4A1 New Series and a CA M15A4 (sorry, no ICS details). Since you asked about what company's M4 to buy let me offer you my opinion:
If you prefer a more realistic looking M4 (more metal especially) then the CA version is the way to go. The TM version has more plastic furniture and so therefore doesn't have the same real steel look & feel. Another consideration is that since the CA version is built with more metal it is the heavier of the two AEGs. This consideration maybe important when you are carrying the weapon all day long in a gaming event, your arms will notice the difference.
If you prefer a M4 with the probability of more reliability then I would suggest that the TM model is better then the CA model. I am not a gunsmith expert by any means but my Airsoft mentor has a small article on his blog that talks about what he found when he took apart a CA M15A4 CQB AEG recently which I believe uses the same internals as the CA M15A4 Carbine. I am posting the blog url here for your review:
http://www.colinsun.com/Colin/nfblog/?p=450
I hope this information helps. I believe that both the TM and CA versions are great AEGs in their own rights but if I had to make a choice I would favor the TM model since a functioning AEG over how it looks is more important to me.