Plextor was the role model during the protected CD/DVD burning days. Meaning they have experience with programming drive firmware, but they are new to SSDs. The Plextor SSD is using a Marvell controller so hopefully they just QA the Marvell firmware then brand it. Crucial on the other hand is a MUCH BIGGER monster in the NAND world. Crucial is the public store of Micron who with Intel developed NAND chips. Despite this, they are new to drive firmware. They also probably QA Marvell firmware and brand it, but their QA is VAST. Plextor's QA may be smaller, but they have historical experience with optical drive firmware. Of course, I'm just guessing. QA can be outsourced. It may also be included in the Marvell controller business package.
Regardless.. If the price is the same, I'd go with Crucial. Crucial's warranty is 3 years and honestly, you're going to end up upgrading well before 3 or 5 years. 🙂