Play both and all romances and the favortisim becomes blaringly, and annoyingly obvious IMO. (I played sexy-voice Italian mshep first, so I'm biased TBH.) The english V.A did his best in In 3 IMO. I hate the over-done 'we fight or we die' by the female actorr, but the english actor nailed it IMO
I can't comment on Italian voice actors, but I don't really thinkt here was a difference in how Meer acted certain scenes with Liz Sroka compared to Ali Hillis. The dialogue is certainly different, but I didn't notice a difference in delivery. Is there any scene in particular you are referring to?
But anyway before this goes way off-topic ..
LOTSB Liara, or ME3 Liara? and why.
I don't think they're radically different, but I'd go with ME3. I'd pick ME3 only because I think Liara is more emotionally balanced in that game. In LotSB prior to the date she's got a single-minded focus on vengeance, which tends to manifest as her being more ruthless there than she is in other chapters. That single-minded focus on vengeance was a little unhealthy for her, which is also why Shepard had to use 'interrupts' to get her attention.

Her portrayal in ME3 is sort of a balance between the more ruthless side of her that we see in ME2 and LotSB and the more shy and good natured side we see in ME1.
ME1 is my least favorite chapter for Liara.