Hi,
I am also interested in this question. Although I don't know the exact details, if you look in the manual section 2.17 "Legacy vs Current Elements" it gives an outline of the general differences between "current technology" and legacy elements. It seems that you can approximate (replicate?) the behaviour of solid45 by using solid185 with certain keyopts.
I think the short answer is that solid185 should produce more accurate results, and has more bells and whistles. However probably for simple analyses the results should be pretty much the same. The manual still recommends that you use the current-tech elements though.