I think as one progresses, or in order to progress faster / more effectively, it is eventually time for most people to get a quiver of the same model sails.
I went ahead in leaps and bounds when I got all the same model sails from the same manufacturer (I use 4.5, 5.0, 5.3, and 5.6 Simmer X-Flex for 90% of my sailing).
It is great to know that when you've reached the upper limit of wind range, you can change down, bung in a mid range downhaul and outhaul setting, and it will feel sweet.
With a mix of manufacturers, you may find that your 5.0 is a powerful model and really a 5.2 in REAL area, and your 5.7 is a less powerful model and really 5.5 in real area... so you may as well ditch one. The reason being the 5.0 produces a 5.3-ish power and the 5.7 does too. Plus you will never discover the best settings as it take many sessions as 5mm changes in downhaul to find the sweetest set. Instead of doing it a few times, you must multiply that "few times" by the number of diferent sails u have.

Only exception for wavesailors I guess is your one big lightwind sail ( even more so for heavy dudes like like me). Because all my smaller sails are short boom, sideshore oriented wavesails, I have a 5.7 Crossover (onshore wave / freestyle) which is a bit more powerful than the 5.6. As 90% of my sailing is down the line in a nasty spot, the 5.7 is for 15kn in up to chest high waves. Any bigger waves and it is a bit hard to depower so I use the 5.6 and put up with grovelling out, but a ****hot better ride in.
Then my 6.4 for real desperation days is a pure freestyle, lots of power but will only use in waist hi waves.

Best thing I ever did was commit the $$$$ to two same boards (92 and 80 EVO's) and a bunch of same model sails

Notwal and bender, yep same here, As long as I like it, Pfftttt I will buy it, but at a price I reckon (I reckon, and not some hype sales gimmick) I will buy.
Don’t mind 2nd hand, as long as they are OK condition and the Forum views haven’t smashed reputation with constant failures, or are real right pricks to set.

Bender, wonder why the Norths are tad better? I have a 2004 4.7 and yep its just that much better. I did buy it new, but at a "its a year or two old, price" and a piece of piss to set.
Not that I am any good at any set ups, so the easier it is the better I like it

Many thanks for the input guys. I am starting to understand maybe why sticking to one sail brand has its benefits. I come from a dingy racing back ground and learnt to windsurf in the 80's on windrush gear. I stopped as surfing took over and have just got sailboarding again. I cant beleive how easy the new gear is to use. I think will be saving my $$$'s and getting a stable of either ezzys of Norths.
cheers guys