Stated diameters and strains are not always correct, but I think many of them are not too far out. I have tested quite a few on my micrometer and find that if they are out, it is usually only by something like 0.05 on a line from 0.08 to 0.20. I haven't tested any for breaking strain, as I don't have a reliable way of doing it.
The odd line does crop up that is nowhere near the claimed diameter. Powerline is the most obvious culprit, but many more traditional monos are understated as well. Although one line did surprise me recently when I bought some for float fishing on a river and that was Bayer Perlon. I used to use this for hooklengths before high-tech lines came on the market. It mic'd up pretty near bang on the stated diameter every time.
There are other considerations than diameter and breaking strain and these are often ignored in this kind of debate. Suppleness and abrasion resistance are the two easiest ones to work out simply by feeling the line and rubbing it over a piece of wood.
Some lines will crinkle badly when rubbed a couple of times over wood, others hardly get marked or burred at all.
If you hold a length of line off the spool and let it drop, you will see that some seem to float down, while others just look stiff. A supple line may be what you want, but there are occasions when a stiffer line is better, when trying to stop carp ejecting a bait, for instance.
Another overlooked point with line is how consistent the diameters are. I used to use Matchteam until Powerline came on the market. I then swapped to Powerline for about a year until I worked out the shortcomings (more later...) and then settled on Sensas Feeling. Recently, I tried Matchteam again, mainly because having convinced the boy Fisk about high-tech lines, he was using it for his barbel fishing with great success. I bought some spools and saw that it was softer than the Sensas line, so made up some rigs. I now saw what I consider to be its downfall - the diameters are quite wildly different as you go through the spool. In some cases I found I could feel much thicker bits as I slid my fingers along a rig.
The other problem I found with Powerline was getting knots to bed down properly. I had lots of them spring open when the line was under no tension. Spade end whippings would slip off hooks and pole rigs would untie themselves. I decided to move on because of that, not because of the diameter discrepancy. Once I know that the line is not accurate for diameter, I can live with it as long as I know by how much it is out.
To finish, the main considerations should probably be: can I afford the line and is it easily available? If you can walk to your local shop or order regularly online, then that may help to decide on a line for you.
I have tried, tested and like the performance of Feeling, as mentioned, Browning Cenitan and Shimano Silk Shock, but I can only buy the Sensas line local to me so that's what I use.
As to being able to afford it, I know that most top match anglers use Shimano Technium for feeder fishing. They may say in articles that they are using their sponsor's line, but it is most likely Technium because it is such a good abrasion resistant line. It definitely gives an edge if you are using it because it is practically unbreakable.
But at £35 a spool, I know I can't afford it!
