Flight Safety
I'm not sure of the criteria for this, but is having more medium tankers better during air operations than fewer large tankers?
Speaking purely from the tactical perspective, and as an MC, personally I would go for several smaller tankers rather than one large. You can always have smaller ones operating 'in-cell', which effectively increases the number of hoses from one refuelling point. This also gives the flexibility to split the cell in support of several widely disparate actions.
Defensively, there are arguments for and against - Tankers are considered HVAAs, and you might increase the number you have to 'defend'; against this is the idea thet the loss of one doesn't take the whole capability away.
In the late 80s and early 90s, there was alot of talk of small tankers (VC10/Victor) operating relatively close to the red line, supported by large tankers to refuel the small refuellers further behind the line.
Unfortunately, from a UK perspective, we are hamstrung not only by the poor serviceability of tanker assets, but also by the number of crews available - from this last point at least, it follows that fewer is better, but that's what happens when the military is run by accountants!
STH
Edited for 90s/80s muppetry!