There are threads on FPN about comparing the Sailor Realo to the (superficially very similar) Montblanc 146. Do a search; I think there was even a review with side-by-side photos. Both are excellent fountain pens but with very different character.
The only striking cosmetic difference is that the Realo has an exposed ink window while the 146's is covered when capped. The two pens are very close in length capped, while the MB is about 6mm longer uncapped, mainly due to a longer piston knob.
In any given nominal tip size (say M) Sailor FPs are much finer than MBs, for example the Sailor M might be comparable to MB F. Out of the box Sailors may write drier, but that can be adjusted. Sailors also have more feedback, but again that can be changed (preferably by a recognized nibmeister, unless the user has a lot of experience in nib tuning.)
The Montblanc 146 is a bit heavier and feels more solid, but both pens have excellent fit and finish. Hope this helps.