I have the Ashley big dot with tritium front and back on a Glock 23. I like them. If I were to do it over again I would probably get the standard dot front sight with tritium insert and the regular non-tritium back sight.
Here is a strange analogy, but with the Ashley sights, the front sight is like the sun, the back sight is like the horizon. Perfect alignment is when the full sun is just over the horizon, just like a good sunset.
Instead of getting a rear night sight, I would apply the money towards getting a rail mounted light like the M3 (of course it depends on what type of gun your getting).