What are the functional differences between a digital sampling 'scope and a digital spectrum analyzer?
Well, you're not going to be making RF measurements up to 30 GHz without spending a bunch of money, so either path is big bucks. 

Typically, Spectrum analyzers are used to do frequency domain measurements. You'll get a display of power vs frequency on the display. The controls in the SA are setup for relevant things, Center frequency, bandwidth, resolution bandwith, signal powers in dBm/dBc etc. 

Digital oscilloscopes don't directly have sampling rates to directly sample a 30 Ghz signal, so they'll undersample and assume that the signal repeats. probably a safe assumption, although with no front end filters built into them, you've got dynamic range issues, as well as aliasing concerns that aren't present in a Spectrum Analyzer. You won't directly get spectral plots out of a Digital oscilloscope, you'll need to do an FFT on that. Now, that opens up a can of worms. FFT bin width/windowing function selection, etc. All stuff that can be worked through, but another question to deal with. 

You won't get eye diagrams out of a spectrum analyzer, it's a useless measurement @ RF. That's a demodulated signal measurement. 

Ultimately, if you want time domain data, then use an oscilloscope. If you want Spectral information, use a spectrum analyzer.