As Noua said, it is not really fair to compare these two drives (The 4120 is dual layer though, 2,4 speed). Being available for some time now, the 4120 has proven to be a reliable drive producing high quality burns. The Plextor still has to prove that.
As you wrote, you don't really care about the price, so the answer is easy ... the Plextor is brand-new so it might 'last for a while'. It has faster writing speed on almost any type of media compared to the 4120. The only downside is that it will not write DVD-RAM, which the 4120 does. You will probably know whether this is important for you. If you want to make it safe, wait a couple of days till the 716A review is up on this site.
If you decide to go for the LG, buy the 4163.