Bottle always. Half-bottles, even better, especially if I'm dining by myself.

A few yrs back, I had ordered by the glass and the house wine was a Merlot. When the glass was served to me, I took a few sips of it and it was obviously not a Merlot (it tasted more like an old Shiraz that was quickly fading). I complained to the waiter and he took it back, took it off my bill and quickly offered me a bottle of something at the same price for a glass. A few days later the waiter called to apologize and told me that when they serve wine by the glass, they usually tell the bartender "red wine" or "white wine". THAT was how they served wine by the glass! It said Merlot and Chardonnay on the menu, whatever was served was up to the restaurant! I was speechless.

After this experience, I have ceased ordering wine by the glass.

I have also been very outspoken about wine pouring and how the etiquette of pouring to the customer and having them OK it, etc. has really faded, even at very formal restaurants. This is one aspect I wish the restaurant industry would take measures to preserve. Wine should not just be a compliment to a meal IMO.