For a boy. These are NOT nicknames, so do not suggest [name]Leonard[/name] or [name]Alexander[/name].
Definitely [name]Leo[/name]. [name]Leo[/name] can definitely stand as a full name on its own, whereas [name]Lex[/name] on its own seems more insubtantial to me.
I also prefer [name]Leo[/name]. I like [name]Lex[/name] too, but think it would often be assumed to be a nickname…not necessarily a big deal.
[name]Leo[/name]. People would undoubtedly think that [name]Lex[/name] is a girl (short for [name]Lexi[/name], [name]Alexis[/name], etc.).
I like [name]Leo[/name] better as well. [name]Lex[/name] makes me think of [name]Lex[/name] [name]Luthor[/name].
I prefer [name]Leo[/name], but there’s a [name]Lex[/name] in my son’s class, and I do think it’s fine as a standalone.
The aforementioned son wants me to tell you “[name]Lex[/name] is better.” 
I much prefer [name]Leo[/name]. It seems more substantial as a stand alone name.
[name]Lex[/name] is all girl to me. [name]Even[/name] as a girl’s name I don’t like it. I can’t get the rhyme Sexy [name]Lexy[/name] out of my head.
Definitely [name]Leo[/name], this can stand on its own but I don’t think [name]Lex[/name] can.
[name]Leo[/name] all the way!
Edited for privacy.
[name]Leo[/name] isn’t a nickname. [name]Leo[/name] has a long and proud history of saints and popes as well as literary and astrological connections. [name]Lex[/name] has - well, nothing in comparison. [name]Leo[/name] all the way!
Here’s another decisive vote for [name]LEO[/name]. Great name!
I am going to have to politely disagree with everyone on this thread and say that I prefer [name_u]Lex[/name_u]! I love [name_u]Lex[/name_u] as a stand-alone name. It is on my baby name list too. I find it to be a solid, masculine name.
I love [name_m]Leo[/name_m]!