If you *don't* get the digital card, you will still have pristine sounding vocals.
But I found the addition of the card meant I had the ability to go totally digital downstream from the card, which in my case was an advantage. Now I run all digital
(Guitar-Pod XT- Digital mixer - recorded track )
(sequencer - synth modules - Digital mixer - recorded track)
(Mic-Focusrite-Lexicon reverb- Digital mixer - recorded track)
When the only analog devices in your entire setup are a guitar and a couple of microphones, the quality is superb, whether you record via USB or via the Mia.
Once you've heard your tracks at 24bit/96kHz, you can audibly detect the drop in quality dithering down to 16 bit/44 kHz on CD.
So basically the Mia and the Focusrite/digital card assist greatly in getting the signal as interference/impedance mismatch/noise free as possible.
I would never sell my Mia, or my Focusrite pre. The combination is unbeatable for my money.
But (as someone else pointed out here) you'll have to ensure your playing / vocals are pristine because every little hole in your technique will be apparent. But you have the luxury of time to get the perfect "take".
Try the analog Mia route first, and I guarantee you'll be happy. The digital route will later give you the satisfaction your signal is about as clean as you can make it, should you feel it necessary to go down this path.
You can always "dirty" a clean track, but you can't always "clean" a track that is unavoidably "dirty". The Rode-Focusrite-Mia path is clean clean clean. I'd be surprised if you have very much hiss at all, unless you boost the high frequencies through the roof.
Another option available to you is running guitar-effects pedals-focusrite pre-Mia. This is not a bad sound at all for those times when it's 3am and you're recording using just headphones.