If the focuser can handle the weight, a 31mm Nagler makes a very nice 19mm Nagler when used with an Antares 1.6x barlow - it has much more eye relief than a 20mm Nagler too. When I use the 31mm alone I flip the eyecup down; when it's in the barlow I flip the eyecup up.
For medium magnification views of nebulae and globular clusters the best thing you could do is buy an 10" or 12" dob.
The 120ed with 31mm nagler will have a washed out but wide FOV in town - still good if you want the wider low power field, but if you want contrast too you need to get to about 60X magnification (15mm eyepiece) but then DSO's are getting a little dim due to the restricted aperture.
If I had your scope I'd use it for widest, low power FOV and for moon/planets if I couldn't be bothered setting up a bigger scope. For DSO's, especially from in town, you really need a larger aperture.
Hope those thoughts help rather than confuse
