This is a famous question for tuba players, but I'm not sure there's a single answer for you; I ultimately think it depends on what you want to privilege. (And before I proceed with my answer, I'll go ahead and say that I'm a tubist who only owns a CC, even though I'd love to own an F! I subbed for some professional orchestras back in the day, but these days my career is spent teaching music theory as opposed to playing.)
But to address your ultimate question, F tubas can go pretty low; with a fifth valve, it can play everything down to that pedal F with relative ease (if with a lot of fingers involved!). The problem is that, even in the hands of a great player, these lower notes tend to sound a little hollow, lacking the timbre that's so characteristic in those notes on a CC (or BB♭) horn.
As an example of this hollowness, check out this recording by Michael Lind of the second movement of the Vaughan Williams Tuba Concerto. Lind is my all-time favorite tuba sound, but check out those last three notes (starting around 4:15); you hear how wobbly they are, and they just lack the "beef" of a typical tuba sound. And believe it or not, this is only the D–C♯–B below the staff!
It also may be worthwhile to check out Lind's cadenza in the first movement. Watch with the music (it begins with the C below the staff) to see the sound that Lind is able to create on these lower notes to see if that's something you're interested in. Just remember that these are the sounds created by one of the all-time greats!
But ultimately, my recommendation is this: if possible, spend a week or two playing all kinds of repertoire on a CC tuba. Then, spend a week or two playing that same repertoire on an F tuba. (You could even meet in the middle and try it on an E♭ tuba; I know of at least one performer/teacher that spends most of her time on an E♭, and she both solos and plays in an orchestra.) Ultimately this is a personal decision, and so that time spent determining which is the best fit for you will be worth the hassle.
