I would suggest that steel is better because
it only get attacked by rust, rather than marine organisms and rot. Rust can be guarded against; for example I have seen steel inside an early (about 1930) steel dutch barge which when protected by a special grease looked almost new. Admittedly the outside is still vulnerable to attack but, given decent ship's husbandry, it isn't too difficult.

You should always get the vessel surveyed before purchase (cost about £5-600) and here I believe steel has another advantage in that it is very responsive to sonic testing (and part of the survey should include a thickness diagram)
You should be looking for a minimum of 4mm (below that and it needs attention) If neccessary it can be overplated - the new plate should be at least 6mm.
The inside frames, unlike wood, are not likely to present too much of a problem.

If you do look at wood then stick your head into the bilges and the back of cupboards etc and sniff! If you detect a musty smell then closer inspection is called for. (Again, a pre-purchase survey is called for)

I hope this gives you a couple of answers for starters - you can always come back and ask for more!