Mamiya rz67 180mm, W-N vs. SB + spacer?

I was wondering if there are any reasons to go for the 180mm W-N instead of the SB + spacer? Do you have to make any exposure compensation with the SB + spacer? Is the finder darker? Is the combo heavier than the W-N alone? Anything else I should consider?

Thanks.

It is heavier, otherwise no difference really.