If you use the half bottle method with an older or otherwise more delicate wine, it's best to siphon to minimize oxygen exposure in the transfer. There are at least a couple products along these lines, but with variable volume, that you can also pour from. One is the Wine Squirrel. Same basic concept as the variable top stainless tanks we use in wineries. Also, along the same lines, someone gave me a PlatyPreserve, which I've never used, but is a cheap bag, so it also offers the variable volume.
The problem with methods that leave head space is volatile aromatic compounds will leave the wine, seeking an equilibrium. So, the impact will depend on how much headspace there is and how much volatiles are in the wine. With the Coravin, talking with various itb people who use it, as well as several forum discussions with regular users provides a lot of anecdotal evidence it works well down to a certain point. In the tasting room, when it's the end of the week and maybe under a quarter volume left the wines can be flat due to lost aromatics. Consumers have said they've been fine sampling periodically for 6 months through the first half of the bottle, then they just pop the cork and drink the rest.
I have Private Preserve on hand and use it infrequently. If used properly you're sparging out most of the oxygen, but not all. So, it slows down the oxidation process. Better than nothing. Refrigeration also slows down oxidation.