Beefing up deck joists: Decrease spacing or increase joist width?
First of all, those load tables are for L/360 deflection, which is already pretty stiff. That means that fully loaded, the beam will only deflect 1/360 of the length of the beam, or in this case about 1/8". If you can find a 480 deflection table that would give you even stiffer numbers.

Having said that, both adding depth and increasing the number of joists are valid ways of increasing the strength and stiffness of a floor. Upsizing to 2x8 would be a lot easier than adding more joist, and in general it is a more efficient use of materials to make a beam deeper than wider (and adding more joists is effective adding to the "width" of the joists). If you want to compare your two options, you can see that 2x8 @ 24" will give you a stronger floor than 2x6 @ 16" by just comparing the permitted span lengths. E.g. if you are using No. 2 southern pine with your loading conditions, 2x6 @ 16" has a max span of 6'-3" and 2x8 @ 24" has a max span of 6'-6". That means that the 2x8 is stronger.

(By the way, 100 psf live load?? Do you live in an area that gets a ton of snow? Or is this a public/commercial building? Typically residential decks are designed for 40 psf. Note that if you are serious about building a 100psf deck you will need to make sure that all parts of it are built with that load in mind... decking material, rim joists, columns, foundations, etc.)