IMO 600CFM are enough, especially if you're not wokking all the time. Given the hood size restriction, I actually think that too many CFMs are detrimental for particulate capture.

The only reason to go for 900CFM is noise-related, as that hood would probably be quieter at 600CFM than the one that maxes out at 600.

There are two issues in the OP's message that deserve some discussion, or perhaps better stated, that deserve a review of innumerable previous discussions on this forum.

1) In performing its task of capture and containment, the hood has to have enough air velocity at the baffles (or mesh filters) to ensure that uprising effluent (about a meter per second for gas cooking) does not "reflect" out of the hood. This velocity has to be present over the entire hood aperture, whether one or more burners are in use. The CFM/BTU rule of thumb is only a crude counterpart to the actual issue of hot surfaces plus hot flame effluent rising upward at some velocity (that varies with angle and temperature). I recommend 90 CFM per square foot of hood aperture based on arguments I won't repeat here. This is commensurate with the 85 CFM/sq.ft. that Greenheck recommends for a particular type of commercial cooking. Such differences are irrelevant anyway in the context of very poorly documented pressure loss and baffle aerodynamics characteristics of hoods.

2) The actual flow rate desired above cannot be achieved with a blower of that same rating. Due to various pressure losses getting air out of the hood and replacement air into the kitchen and passing across the baffles, the rated zero static pressure blower specification must be higher than the requirement for CFM when cooking. I recommend either a careful HVAC analysis (which unfortunately will be limited by the lack of good data for the hoods and data for the house leakage or MUA path) or using a factor of 1.5x the desired CFM.

For example, a 2 ft by 2.5 ft hood aperture has an area of 5 square feet. Multiplying by 90 yields 450 CFM. To get this may require a roughly 675 CFM rated blower. So, if there is reasonably low-loss MUA available, a 600 CFM rated blower may be adequate. If the house is fairly restrictive, a 900 CFM rated blower may have a better chance of achieving the desired 450 CFM.

Note that all the rising and expanding effluent that misses the hood will not initially be captured and contained, and only house air replacement will remove that part of it that hasn't condensed on the walls and ceiling. Using an excess CFM rate (relative to the analysis above) will have the effect of slightly increasing the effective hood aperture, so this could be an argument for going toward 900 in blower CFM rating. Usually, where space allows, using a larger hood is the preferred approach to good initial capture.

kas