Im looking at getting myself an Eotech for my AR15, but I'm not really sure which model I should go for. First of all, my AR15 has the fixed front sight. I know you can use both absolute co-witness and lower 1/3, but which option do you prefer? Also, is the XPS/EXPS series bring anything more to the table than the 512/518 besides just size? I don't intend on ever using a magnifier, so I don't have an issue with the bulkier profile. I'm just really new to these sights, I'm used to scopes, so I want to make sure I get the right one for me as they're not cheap. Thanks.