If the choice is between those two i'd choose the Whidden FL bushing sizer, they are of good quality.
If you do not use a expander in your die, you should seriously consider turning necks and get a expander die.
Having all the neck inconsistencies, pushed to the inside of the neck by your bushing means you will not get the neck tension you try to achieve with your bushing, and it will increase runout in some cases.
I do believe that a correctly sized expander mandel in a expander die that lets the mandrel float, and a O-ring under the die (if not a CoAx) will achieve the best and most concentric results .
Something you should consider trying with any conventional press and and dies too, i only know of one where it has not helped reduce runout, and it costs you maybe a dollar or two.
Used with a bushing in the sizer that keeps the neck at least .001 tighter then the expander mandrel is what you should aim for.
If you choose not to turn necks, with your setup that is the ideal solution, if your looking for consistent neck tension.
If you start mixing brass you will need several of those of course for each type along with the correct bushings for each type of brass.And for most brands you need a diferent neck bushing any way so it adds up 15-15 dollars for each type of bras you want to use.
As for custom dies and later use, when you change barrel witch will happen sooner or later, the reamer will usually be different, so you can be fine. But you can also have a die that oversize the brass, and reduces the case life sometimes quite a lot.
Or you can even have a die that does not size the brass enough and makes it harder to chamber.
To a minor or larger degree one of the above mentioned will be correct.
If you have a factory barrel and chamber, and will replace it later with a custom one, if you want ideal performance and long brass life a new die might be in order.
This you can easily compare by measuring brass shot in a new chamber Vs the old, get reamer prints or do a chamber cast.
The ideal solution for custom dies, is buying a quality chamber reamer and prints. And have the same gunsmith chamber the new barrel for that rifle every time with your reamer. Prints can be used for dies and to order new reamer from the same mfg, if it gets worn.
Use the same brand of brass and turn the neck to a consistent diameter.
That will give you custom dies that will last you a long long time.
A WTC die witch is a piece of art, you could order up a different die body insert, and roll with a new chamber no matter what dimension, and of course use the old neck/shoulder bushings if neck diameter was still the same, or get new ones.
Compared to a custom Whidden it is very expensive though, and has a 1 1/4-12 thread pitch.
As for brass sticking to one brand except for the neck issues it will usually work but your optimistic if you think all types are identical in size and volume. And the volume difference will affect pressure, going from Hornady to Lapua might be okay just be careful with hot loads, as a new load development will often be needed, and for optimal performance it certainly is, as you can expect it to affect your OCW.
Never used Hornady brass other then factory ammo i have shot once, and do not shoot a 6,5 Cm so can not give you a true answer to your question.
I would worry more about case dimensions and volume then brass springback (soft/hard) really.
