Well, where to begin? I suppose the first questions are what do you want the bike for and how much do you want to spend. The second thing is that if you are looking for performance such as for racing then the only sensible choice is carbon.
Now to your question;
In some ways, titanium is the ideal frame material. It has a good strength to weight ratio and doesn't rust.
However, it's relatively expensive and custom options are limited as most titanium frames are not made in the UK. Another issue is frames cracking. I don't know how common this is but plenty of reports on this forum. I believe it's down to poor quality control in far eastern factories. Still, if your want is road, audax or light touring, there are plenty of titanium options.
Steel does have its downsides. It is relatively heavier and can rust (except for stainless steel). However, where it comes into its own is versatility. There are still plenty of British builders around so you can get whatever you want if you are prepared to pay for it. There is nothing better, in my view, than sitting down with a frame builder and cup of tea discussing your frame, then returning several weeks later to collect it. If world touring is your thing then steel is a must. If you are stuck in the middle of nowhere with a broken frame then there's a chance a local welder can fix it. With titanium, that's unlikely.
Finally, a word about forks. They come either in carbon or steel (I have seen aluminium forks but they're crap). A titanium frame will probably have carbon forks which limits your load carrying capacity. A steel frame can have either steel or carbon forks depending on what purpose you have in mind.
Personally, I love steel. Five of my six bike are steel, the sixth being a Brompton with titanium bits.
Well, that's my rant over, hope you find it useful.
