I've used Harry's Lap timer with my iPhone but ended up going back to RaceChrono with Android. I had a used Nexus 5 to use RaceChrono and using that as a dedicated realtime lap timer/monitor seemed like a good idea. Last time I checked it was not currently available in Apple Store.
I like the RaceChrono over the Harry's Lap timer for several reasons; instant lap delta with previous best and ability to export the whole session instead of each session, more accurate POIs. Also for realtime analysis, I liked the fact that it can use the OBD2 data integrated so monitoring some information is possible and looks a lot better in RaceChrono such as speed graph with satellite map view.
I've been using RaceChrono with the 10 hz external GPS (SkyPro XGPS160 GPS Receiver)