Based on my taste I would disagree with many of the replies and say the Speedmaster, since I find the IWC design much too busy. However, if you're looking to connect with your Grandfather's history through the watch and you think the IWC would do that then opt for that one. A purely practical question - have you tried the spitfire on a bracelet or would you want to? I find the Speedmaster can be nicely dressed up or down but the IWC looks out of place in a dress environment (obviously just my opinion). Lastly, if you're not wedded to these two as an option, have a look at the
CWC 1970 RAF PILOTS CHRONOGRAPH which might actually be closer to something your Grandfather would have worn. Either way, keep us updated and enjoy the search.