I used to use an underarm digital one. It was such a pain coz you had to hold it there for one minute to take the temp. Matthew did not like it and would scream and squirm and it always came out no matter how much I held his arm down.
I went and bought a digital ear one (cant remember the brand) and that very day he got a temperature that lasted for 6 days. He was back and forth to the hospital, so we were taking his temperature heaps. It was always in line with his temp when it was taken at the hospital. I would never have been able to keep such a close eye on him with the underarm one. His temp hit 40.3 at one point, so it was good for me to test him every half hour.