Full water changes are required if you have gravel in it. Dirt will definitely accumulate within the gravel bed.
As for no gravel, I don't think a 50% water change will differ VERY much with a full water change. In small tanks just like yours, a 50% water change will greatly alter water parameters making your tank go through another "mini-cycle". In my case, I do full water changes weekly, but I make sure that my filter is mature(filled with good bacteria). It's much easier for me and I haven't had any troubles with this system. Just make sure that the water is aged and treated properly. In HMs(Half-moons) in particular, I find that this regimen works best, because with the long fins of an HM, even a little dirt on the ground can cause havoc when my betta is resting on the tank floor. In conclusion, it depends on what will work best or more convenient for you, try to mix things up, you might find other things that could work better for you :)