Providing you buy the correct size and specification then both a steel or a Titanium frame should give you good service and last a long time.
Traditional materials used in frames hand built by traditional craftsmen are still a valid long term investment. It is more a case that modern frames with modern materials are more about engineering than craftsmen ship, so no longer do you need to go for hand built to get high quality.
To an extent it is horses for courses, I ride both
Steel Audax and
Titanium Audax framed bikes, the latter replacing the red steel Dave Yates built Pearson shown, as I could tell it was rusting from the inside out quite badly. I still use the blue one for work and for winter miles so am able to compare them. Both are quite similar to ride, if I had to highlight one difference I would say the titanium is definitely the more comfortable of the two, it is almost like the roads have been resurfaced, it feels smoother to ride, a comment often made by those who ride Titanium. That is not to say that I find the steel one uncomfortable by any means as it certainly is, but a difference there is.
In conclusion you are looking along the right lines for a suitable bike that you intend to keep a long time, in the end it is your choice; I doubt many will have anything negative to say about either and rightly so.
Paul_Smithwww.bikeplus.co.uk