I think this is a question that does not have a definitive answer. It’s all based on preference but after trying both types of plates I have found some pros and cons of both. Lets start with photopolymer (pp). If you have a design with fine lines (o.75 or less), the pp will not really hold a line that well. Meaning if you intend for the line to be .5, once you print it it will turn into a .8 line. That can really mess with ones design. If you do not have fine details to your designs then you don’t have a problem. Also, the sticky backing can be kind of a nuisance. Either the pp slides or it is so sticky you cant get it off your base. After repetitive pressing the pp can be quite tough to remove. I have to say, since the pp is transparent, the registration is really a cinch. That in itself may be the reason I might stick with the pp.
Now the Magnesium. Magnesuim will give you a crisp image at pretty much any resolution. If you want a .25 line then that is what you shall get. Also, (I might get some rebuttal to this) I feel I get a deeper impression with the magnesium plates although, Boxcar Press has addressed this problem by making a deep relief base as well as deep relief photopolymer (for more information regarding bases and plates read our post on Making your own base…). Registration can be tough with the magnesium since they are mounted type high on wood and unlike pp it is not transparent.
I guess you might have to try both as well to see what you prefer to work with. Happy letterpressing!