Cork or stopper, kill it in a day or two, no more than three or so. C'mon, it's only 4-5 glasses!
All the great tools out there to preserve wine (high end stoppers, air pumps, neutral gases, etc.) are basically gimmicks. Wine starts seriously degrading the minute it's opened, sparkling wines even more so.
It remains utterly inexplicable, but a champagne/cava can be saved for the next day by placing a silverware or steel spoon, handle down, in the bottleneck, the spoon end still out of course, and replace in the refrigerator. Next day, the bubbles are still there and the taste intact. If anybody knows the science of it, please relate!
I like to keep it all under the $10/btl range for the daily drinkables, usually assisted by the 10% case discount most stores are offering these days. If you want something not available in that range, such as a Barolo, Gigondas, etc., one has to go higher, and I have settled on a nice Russian River Pinot Noir [Sonoma, CA] winery (Merry Edwards)for supplying my secret pleasures that I buy on occasions. I try not to use it except for special times, and still have a few stashed in the basement. When Richard Lindquist, BansheeOne and Dave Clark arrived in Seattle at the same time at the airport for last year's I&I, we repaired to my place and killed off a couple bottles of Cote de Rhone I think it was.
As Michael Eastes commends, <Que sera, sirah>!