Vertical, its the shorter way around the circle, especially if you start your Immel from under their nose as they are diving towards you (they are gaining E at this point which will extend their turn radii). Keep enough E to afford a second Immel, if necessary. If the bogy zooms, forget the second Immel, extend, regain your E, and setup for scissors/rolling scissors as you now have a BNZ fight on your hands.
I usually let the bogy look at my 6 since he/she has already proven they will just zoom in a head on engagement, head on being in front of the 3/9 line. Then try to sucker them into scissors/rolling scissors. Why scissors? 'Cause my ride of choice is the FW

(It rolls the fastest and bleeds E like a stuck pig, the two best qualities for a successful scissors.)
A vertical FW will be pointed to the other side of the circle (that's usually where the bogy is, btw

) before any flat turning ride (exceptions being Zekes and Hurris, but then you just zoom them, or remember earlier "Keep enough E to afford a second Immel, if necessary.")
There are a high percentage of people who think most any other ride will out turn the FW, and they will in a flat turn, but fortunately the flat turn isn't the only way and again a well flown vertical FW will beat almost any other ride to the other side of the circle.
One other advantage to flying the FW, cannons. Just a pinch between the cheek and gum (fuselage and wingroot) and BOOM!

In other words, its a great snapshot plane and generally what goes down vs a FW comes back up. If they decide not to counter with an Immel of their own then its time to run 'em down and saddle up or head back over the top. This is where it is handy to learn if your ride can perform a second Immel and if so what speed is needed to do so?

Happy Merging!