30 lb Geared Bicycle vs 20 lb Single Speed, Which Is Faster for Commuting?
To answer this question you have to factor in all the specifics of your commute. In particular those are (including yours for completeness):


Length of your commute (in time - distance is less relevant).
Steepness of ascends (and descents if you accelerate down-hill).
Overall 'windiness' of your city.
Number of full stops and slow-downs you have to do (due to corners, crossings and traffic lights).
Your physical strength and general fitness.
Amount of time you are willing and able to spend maintaining your bike.


Number (1)2-5 need to be considered when choosing your gearing. If the ascends are steep (or you have expect strong headwinds often) you'd need to run a smaller gearing. That would enable you to power through the ascends and accelerate quickly - but it would also limit your top-speed. Also your long-time endurance would be worse on a high-cadence gear-setup.

Meaning: in such conditions it would probably be better to ride a couple of gears. 200%-300% difference should suffice - a range that you cat get out of a small road-race cassette.

If in turn your commute consists of shallow ascends and descends, and you pack enough strength to decently accelerate the gear-ratio you'd want to be running during your average commuting speed - then you can do just fine on a single-speed bike. I rode a commute of 27km (~17mi) on a single-speed bike for years without trouble.

If you're still undecided you might want to consider my point 6. Maintenance can be a hassle, costing time and money. But it is not necessarily much more expensive on a geared bike. On a single-speed bike you have much higher wear on your chain ring and the sprocket, due to the higher torque you put into your drive-train. On a geared bike that wear is considerably lower, since you can down-shift to accelerate, and it is spread over a number of sprockets instead of just one.

My personal choice for commuting: Internaly geared hubs

You can combine both of those upsides by running an internally geared hub. Something like the Shimano Alfine 8 should be more than enough for all commutes, and doesn’t cost more than most other geared drive-trains. It’s insanely durable and needs very little maintenance. I rode one for approximately 25-30000 km (including a technical trans-Alps tour with 20000 vertical meters) and it’s still going strong in my daily-driver commuting MTB. Of course those upsides are bought with a little lower transmission efficiency and a  higher weight. But that shouldn’t be to bad for a commuter. Plus you get instant-shifting capabilities – even without pedaling! This is something I grew quite fond of, because it enables me to brake and shift later – makes it easier to keep the speed high.