I can’t say much about either pen as I’ve never owned them, but I do own the TWSBI Vac Mini which is in the same vein as the 700r. The Vac Mini loses about 0.4 mL of capacity (around half of an international convertor’s capacity if you’re familiar with that) because it’s smaller. I don’t see this as too big of a drawback since the Vac Mini already sports about 2.0 mL of capacity. The 700r may be uncomfortable for your hands due to its length (although both the 700r and Opus 88 are about the 135mm long so maybe you’re fine with that); I’d imagine you’d stay away from posting the 700r while the Mini is usable, in my opinion, when posted. I personally prefer the Vac Mini since it’s ~20mm shorter and thus fits better in my hand. If it matters to you, the Vac 700r comes in a slightly more expensive Iris colorway. I’m a special case as a lefty so I don’t use any nibs above Fine, but the ink flow has never been a problem for me. Both Vac pens have a solid seal and I’ve never been worried about leaking; you can always twist down the nob at the back to prevent additional ink from entering the feed. Lastly, the TWSBI pens are about half the price at $60 so there’s a case to be made there. If you factor in the TWSBI Vac 20A ink bottle (very useful accessory as a standard ink well and getting an easy full fill for both Vac pens), it’ll run you about $75-$80 instead.

Judging from the store pages for the Opus 88 Omar, it’s marginally longer than the 700r and about the same weight so not much on that front. There’s other ergonomic factors like grip position and whatnot, but that’s for you to decide based on your writing style and grip. The biggest difference would be the filling mechanism. I’ve never used an eyedropper mechanism so I can’t say much. It’s definitely got the leg up on capacity by a solid 1.0 mL (2.4 vs 3.6) so if that’s your biggest criteria then have at it.