Dear Alya. I think Micro silica. As microsilica contains higher percentage of SiO2 than metakaoline and as such it will be more effecient in terms of pozzolanic reactivity and consequently it will result in higher strength in addition, microsilica particles is spherical in shape and when being added to water a rolling performance will occur resulting in higher workability than the metakaoline that has flakey shape particles with higher water absorption behaviour due to its texture that will reduce the workability of the concrete mix . In terms of durability. I think microsilica will also be more effecient for being of higher pozzolanic reactivity that results in denser, well compacted matrix with high CSH content and with higher Ph value resulting in increasing the passivity around the reinforcement and increasing the corrosion resistance and eliminating the carbonation effect as well. Thank you
In terms of strength, both can performance similar. Metakaolin has excellent pozzolanic potential similar to micro-silica even with lower fineness. In terms of durability, increase in the amount of CASH due to alumina from metakaolin can increase chloride resistance with additional chloride binding.
Some paper which can be useful:
Article Effects of metakaolin and silica fume on properties of concrete
Article Effect of Metakaolin and Silica Fume on Rheology of Self-Con...
Article Study of effect of Silica Fume and Metakaolin combinations o...
Conference Paper The effect of silica fume and metakaolin on Self-Compacting ...
Conference Paper Effect of Silica Fume and Metakaoline in Consistency and Set...
Both the materials have good potential for concrete production. w. r.t. strength, workability and durability. But depending upon particular application of concrete (Where it has to used) and availability of this two materials w. r. t. economical criteria need to be consider.
You will probably find the textbooks below very useful:
Siddique, R. and Khan, M. “Supplementary Cementitious Materials”, Springer, 2011.
Ramezanianpour, A. “Cement Replacement Materials”, Springer, 2014.
Thomas, M. “Supplementary Cementing Materials in Concrete”, CRC Press, 2013.
De Bellie, N., Soutsos, M. and Gruyaert, E. (eds) “Properties of Fresh and Hardened Concrete Containing Supplementary Cementitious Materials”, RILEM State-of-the-art report, 2018.