I know this isn't related to either Serrano or Las Palmas except as an alternative, but have you considered the new Woodbury complex that just finished construction? I signed a lease there recently, and it's actually a little cheaper with the move-in incentives. The apartments are also brand new (of course), and the fixtures are better. Layout of the 1-bedrooms (at least) is similar, and it's only a little further down Irvine Blvd. I looked at Las Palmas, Serrano, Somerset, and then finally made my way over to Mirasol.
In order of nicest to most basic, I'd say Somerset, Mirasol, Serrano, then Las Palmas. Now, I'm not trying to make any judgements or anything, just an observation in terms of carpeting, fixtures, newness, etc. just as one would rate houses. Somerset was also the most expensive (crown molding, two-tone paint, tile-counter, fireplace!). Mirasol the cheapest (again, move-in incentive included; stainless appliances, crown molding, two-tone paint, granite counters), with Serrano and Las Palmas neck and neck (laminate counters, no crown molding - more basic but still decent). For a 1 bedroom, 750 sq ft, I think the prices avg near 1500/mo, 1400/mo, then 1430 to 1450 for the respective apts when I went looking a couple months ago.
$.02