I enjoy using my Nikon D60 for macro (using sigma 105mm 2.8) and wildlife (Nikon 55-200mm) and having been really pleased with the results. But I am considering upgrading the camera body for the following reasons: ability to shoot at higher ISO without compromising on image quality, having live view and an articulated screen to make it easier to shoot macro in awkward positions, ability to shoot video, higher resolution for cropping, quicker focus speed for birds, quicker access to controls without diving into menus, and ultimately better image quality (though I'm unsure of how much an upgrade would help this compared to a new lens). So Im between the D5500 and D7100 (both used), I like the quick controls of the 7100 and from what I've read it seems recommended, but I really like the articulated screen of the D5500 and it is much lighter (though if I have a heavy macro lens on this doesn't seem super relevant).
Any advice on which may be more appropriate (even if I should consider an old used full frame like a D600), or whether an upgrade would not improve image quality much and I should instead invest in a lens such as sigma 100-400 which would be great for birds.
Thanks for any help :)