welcome

out of the blue question and probably needs more info like room size and spl levels you want to monitor at

but
gut feeling

smaller room with the sub ... 6 inch

larger room with larger sub and louder levels .... 8 inch

or without sub .... 8 inch
that is for the time you want to check without the use of the sub

Is the xover point for the tweeter different for the two ?
If the 6 inch has a higher xover point than the 8 ... then the 6 will have more voicel without the influences of xover point.

this is all just words

Get both in YOUR room and make a judgement based on sound you hear.

Kev wrote: welcome

out of the blue question and probably needs more info like room size and spl levels you want to monitor at

but
gut feeling

smaller room with the sub ... 6 inch

larger room with larger sub and louder levels .... 8 inch

or without sub .... 8 inch
that is for the time you want to check without the use of the sub

Is the xover point for the tweeter different for the two ?
If the 6 inch has a higher xover point than the 8 ... then the 6 will have more voicel without the influences of xover point.

this is all just words

Get both in YOUR room and make a judgement based on sound you hear.

Thanks. I understand that at the end of the day it will be a personal judgement call based on my ear, but I'm hoping to pry deeper into more of the objective technical differences that exist between the two, and how those differences will affect this particular setup and monitoring methodology.

The monitors will be 4-7 feet from the listener, in a large treated room, roughly 16' x 22'.

I'm not sure at what SPL level I will be monitoring at, but being unavoidably vague, I would say at a moderate/medium volume level, maybe sometimes high.

The crossover of the 6 is 2.6kHz, crossover of the 8 is 2.4kHz. Is this considered a major difference? What exact effects will this difference in the crossover have?

2.4 to 2.6 is a marginal difference and possibly not one to base a judgment alone

a higher xover for the tweeter will give it more power handling and potentially more volume
BUT
the tweeters in this type of unit are probably attenuated with the mid driver being the less efficient and therefore limit the max volume.

A higher xover for the Mid/Bass gives it more of the voice/vocal in this one driver ... same goes for a lower xover point here.
Theory is that the voice will sound more natural with one driver handling that range of frequencies.

As for higher SPL
Cone excursion is what limits the max SPL and a higher xover point will reduce the excursion for a given volume.
It would probably take a half octave or octave for a significant difference.

Given these speakers have the same tweeter driver and you are using the same Sub unit
... it is the Mid/Bass driver that will be giving the package a different feel.

I think you just need to get these things in your room and give it a listening test.

Technically these things are probably near enough that there is nothing to chose
apart from physical size

if there is a size issue then the 6 inch may attract you.
With a 16 x 22 room I think you can look at the 8 inch or larger
and so have the OPTION to use them without the Sub.
I like that option.

just my ramblings at 7:15 on Sunday morning ... that time reference at the top seems to be a hour out ???
must be a daylight savings thing ?

you say its a treated room, with what?? a bunch of Auralex on the walls?

the room is going to dictate what you have for monitors. my room has been bass trapped to HELL because of the dual 15" Westlakes that were here prior to my arrival at owning my place. the B&W 808's worked OK in here but the dual 12" didnt cut it so were going with what the room was designed for dual 15" in each cab. so,.....you need a sub to reference other instruments that dont belong in the sub sound field, so,.......what dies your room consist of?? my dynaudio BM15's needed a sub in my control room, whereas my old control room the BM15's were plenty huge.
so, ......... what is your room like, did you ever have it tested, if not you may be wasting your money.....

Personally, based on the information provided, I would go for the 8's. I have the 6's without a sub, but since you have a large room and will be further away from them you will most likely need a higher SPL output. The slightly extended bass range of the 8's would also hlp you when you drop the sub.