One thing that often confuses people with the WA lines is that the listed lenght (not to mention weight) is WITH the included floating tip. Subtract this off and without a tip the WA45’s, especially the lighter weight ones, are basically no more than a longish skagit length head. They cast very differently than a skagit, but this is a reality check on the length. Because they have more slender, refined front ends they are more finicky than a skagit head with respect to tips, but with the right tips (around 8-9 gr/ft seems optimal) both sustained anchor and airborne anchor casts are easy as pie with either lenght tips, and on either the 45 or 55, though doing SA casts with the 55 is a little easier with a 13’ + rod. Like Nickels I prefer AA casts with the 55 lines, but have no problems with SA casts with a 13.5’ rod with the 7/8 and 8/9 WA55s. But those of course resemble more old timey, less compact style SA casts with longer lines.
What is the right weight for your rod? Personally I like those lines about the same weight (again, as listed including the floating tip) as the skagit head that I like on the rod I will use with them. Some people like them a little lighter. Obviously you don’t get a ton of weights to choose from, but I’d probably get one in the range of 3/4 to all the way between the scandi and skagit weights you like on a rod, if you already have a good idea what those are for you. So I’d say for that rod you are probably looking at the right weights, at least from the choices you are given.
In a pinch it is totally legit to buy heavier and lighten them to preference by cutting off lenght from the back if you are comfortable with the operation or know someone to do it for you. In this case that is exactly how they get the different weights at the factory. The back end of these lines is just a long level section, and the front ends are all the same for each weight of the WA45, and again for each weight of the WA55.
Hope that is useful information. What is the best one for you is entirely up to you. But I agree with Nickels that if you prefer, or plan to do more SA casts than AA probably the 45. If the opposite then the 55. If you really don’t like doing single speys and snake rolls then on that lenght rod I’d say you might prefer the 45.
Note: not to put too fine a point on it, but the first thing I’d do with a new WA is weigh it. I’ve heard things may be a bit better now, but there have been large variations in the weight sometimes. Better to do this to get calibrated and as a reality check right from the beginning. I’ve got a dozen or so WA and FFs with multiple copies of some on the “same” lines and the all have their own ziplock with their own, highly variable weight written on the outside. Glass is half full, you can look at these as the intermediate weights they don’t actually claim to offer.