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FiOs certainly offers much higher speeds than Mobile Broadband, especially on the uplink side. In addition FiOs probably has considerable capacity for growth. The bandwidth (and consequently capacity) for Mobile Broadband is more limited, and as more and more high speed mobile devices make larger and larger demands on the Mobile Telephone networks, network congestion isn't a matter of if, it is when and how bad.
If you are talking EV-DO service from Verizon, it tops out at around 3mbps on the downlink, and about 1.8mbps on the uplink according to my reference, although your average speeds are large data transfers are likely to be considerably lower.
By contrast, a typical FiOS connection these days is probably 20mbps down, and 5mps uplink, and on large data transfers, I often see performance that is very close to rated speed. If you are willing to pay for it, you can actually have 50mbps up and down in most areas.
The other important difference is most Broadband Mobile plans have limits on total traffic with charges for exceeding the capacity that can be quite significant. By contrast, FiOS effectively has unlimited total traffic included in the standard price.
I haven't had any issues with FiOS reliability (my DSL however was dead if I could hear thunder), nor does weather seem to impact the performance at all.
What you haven't mentioned is which/whose broadband mobile plan are you comparing to which FiOS plan. The low end FiOS may not be materially faster than your best broadband mobile connection, the upper backets of FiOS (some of which are not cheap) will leave mobile broadband in the dust.