Personally I'd go with an HPI, especially if it is your first car. HPI's can be a bit more time consuming to put together, but the instructions are much better, include pictures for everything and you will be more satisfied with the overall fit and finish of the kit. I have put together both a tamiya and an HPI kit and I was much more satisfied with my HPI.
Also there is a greater aftermarket for HPI's, not by a landslide, but still greater.
The other main difference is that HPI's are belt drive and the Tamiya's are mainly shaft drive, both have somewhat different advantages and you should read up on that.
Pick up a few issues of Radio Control Car Action, they are the gods of RC and you will learn much from them, grasshopper.
If you are going with a touring car and/or a 4wd/AWD then try to get the Touring Car issues.