Like @REI-AlyS , I would recommend The Camaro Blacktec Wetsuit. It is a suitable thickness for what you are going to do in Portland.
From my personal experience going through many wetsuits (mainly surfing in northern California), you would want to less than 2mm on the sleeve for swimming. Thinner like 1.5mm is better for the range of motion. But, it can be cold, depending on the quality of the neoprene.
The most challenging thing about choosing a wetsuit is fit. Each manufacture fits differently, even with the same size. So, you may need to try more than a few. It is like picking a running shoe. This is the reason why you will find a very expensive wetsuit with Japanese neoprene. They are stretchy and warm. While we don’t offer those wetsuits at REI, they are worth every penny if you like to swim outdoors regularly. You could also consider a sleeveless if you are going to swim in summer. Once you find a right fit wetsuit, get two; that way, you will always have one that is dry!
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