Is it more beneficial to romance liara, ashley, or ask to have them both
If you try and ask to have them both Ashley will break off the romance. Liara is probably the best option because you get to spend time with and romance her in ME2. By the end of ME3 it seems like a fuller romance experice story wise.
Liara because Ashley hates shepherd in ME2
Up to you. No one's stopping you.
If you ask to have them both, Ashley will get pissed off and leave you with Liara. As for the romances themselves, just pick whichever one you like more.
"Beneficial?" Neither one gives you a gameplay advantage, if that's what you mean.
What do you think?