How do you compare redhat and debian , what is their merit and weakness ? which one is better ? thx

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Both are supposed to do what the user expects them to.
Red Hat does not exist any longer. Debian does.

Are you refering to Red Hat 9 (and previous) or Red Hat Enterprise LInux ? - Red Hat 9's unsupported now........ whereas RHEL is a paid-for enterprise-oriented current distribution (you could always use a free-clone of RHEL such as CentOS though) - Debian's an excellent, free, multi-purpose distribution (I'm running both Sid and Sarge) - there are a variety of ways of installing it - one being the efficient 110 mb (roughly) netinst.iso - nice quick download, straightforward installer, on reboot set your accounts up etc - if you want to go very minimal you can avoid the package sets and simply apt-get X and your favourite WM from a root prompt after that - once you're up and runing, Debian's a breeze to keep updated with apt-get and you'll have access to a vast number of packages