Have a look at chapter 1 of this guide:
http://supcomfaguide.uw.hu/It explores in detail which way of upgrading the fastest is.
T1 > T2 > Mass Storage > T3 is definitely the most efficient way, but whether getting all T1 extractors to T2 first before building mass storages is better than building mass storages around a T2 mex before upgrading another T1 mex to T2 depends on your situation.
Generally, upgrading another T1 mex to T2 is better than building mass storages around an existing T2 mex since a T2 mex gives you +4 mass for 900 mass and 5400 energy while 4 mass storages around a T2 mex give you +3 mass for 800 mass and 6000 energy.
That's 225 mass/1350 energy per mass for a T2 mex and 266 mass/2000 energy per mass for 4 mass storages + engies will have to get into position to build the mass storage.
However, since T1 mexes are always under risk of getting destroyed, it can be a better idea to put 4 mass storages around the mexes in your base rather than upgrading all T1 mexes to T2 first since the difference is not that large.
If you want to eco-whore like a baws, try to upgrade all your T1 mexes to T2 before building any mass storages, though.