How are you defining "better"

Cost?
Operating issues?
Risk of water ingress?
Do you get snow? A lot of rain?


Single floating roof is more usual unless you're really concerned about water ingress or get a lot of snow or rain.

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Yes, please define "better"
How about evaporative emissions?
A steel dome of that size will be costly.
An aluminum dome will be taller are not as easy to walk on and they have a tendency to leak.
The least expensive fixed roof will be a standard column supported cone roof, a single support column is well within your size range and can be used for other purposes.
If you are needing to minimize emissions, an aluminum internal floating roof suspended from a steel cone roof is well worth looking into.

site is Bahrain, so no snow, not much rain and no specific restriction due to vapor loss.
My opinion is is that single deck floating roof is sufficient for gasoline/MTBE. There are gasoline tanks in the farm with external single deck floating roof, but client who is from U.K, says they have internal floating roof there, plus he is thinking that by doing so in future they can store other things like methanol and ethanol.

So my first point is that for gasoline/MTBE with no snow or rain, external floating roof will do the job? what do guys think.

Plus if i am not wrong there is a method given in API Manual of Petroleum Measurement Standards Chapter 19.1, “Evaporative Loss from Floating Roof Tanks”. I havent gone through it but do we select internal or exxternal floating roof based on the result given here

Gasoline I can't see an issue with EFR in the ME.

The others are very hydroscopic and EFR doesn't allow the space above the roof to be inerted or kept dry to prevent contamination of the product through the roof seals.

UK we tend to get a lot of rain and hence EFR for products sensitive to water can be problematic.

Also a lot of the tanks are quite old and have been retrofitted with internal roofs to reduce vapour emissions due to newer regulations which weren't as strict when the tanks were first built.

Evaporative losses need to be calculated based on the product, storage temp etc. but EFR should be good to reduce any losses to very low amounts.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

The owner can do anything they want within the local and state rules and regulations. The choice comes down to what is the best for the owner. There are many considerations. There is no one "best" unless you pick a criteria or two, which you have not shared with us. Several of these criteria have been listed above. The sun and UV exposure will reduce the seal life and increase the emissions. Emissions have safety, odor and product loss components. Everything has costs and benefits.

You can use MPMS 19.1 or US EPA AP-42 / Tanks4 to estimate losses if your jurisdiction accepts those methods. You can run the numbers many ways and one way will be the lowest emissions but you still won't be able to choose without knowing what you and the owner value most. This is not new ground you walk upon - what have others done in you area and why?

Considerations include but are not limited to:

Product compatibility now and future
Pride
Safety
Environment
Capital cost
Maintenance cost
Replacement cost
Operational cost
Longevity

IFRs - Great post.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.