I'm training for an open water swim in August (2.1 miles). Due to the water temperatures, they require a wetsuit. According to their website, the water temperatures normally range from 63-71. The wetsuits I'm looking at come in the following thicknesses:
5/3/1.5mm or 3/2/1.5 (front/back/arms & shoulders)

With that 'normal' range of temperatures, any recommendations for suit thickness?

Thanks....

Most triathletes and I all have Xterra suits. I have their sleeveless suit with the varying thicknesses and have swam comfortably in early April and late October in Texas.

I have used several 5/3/1.5 in various conditions and they have worked well. If I was going to do several borderline warm swims I would go thinner but not just for a one time deal

Hell of a username. Good job.