As a previous Istanbul local, I can tell you that the Conrad is in a much better part of the city compared to Old Town. I suppose staying in Old Town would be practical if you're visiting for a very limited time and your priorities are to see the top tourist attractions such as Topkapi, Blue Mosque and Hagia Sophia. (The other Hiltons are not geographically far and public transport options are plenty, but Istanbul traffic can be horrendous at times.) For everything else including shopping, dining, night life, and just getting a feel for residential life, Conrad is the better choice. I also don't know if I'd deal with the hassle of moving hotels when you're staying just three nights.