I was priviledged lately to be able to speak at length with a transformer design engineer of considerable experience from California. He informed me that where he is employed, the cost break point for circular disk vs. rectangular sheet wound units comes at about 69KV and 20 MVA. Above these levels, the circular disk design has a slight advantage price wise and a significant advantage performance wise. Where I am currently employed, we have both types in service and speaking strictly from the performance aspect, I greatly prefer the disc or circular wound unit. The natural thermo-siphon action of oil and hence heat dissipation in a properly designed power transformer (disc wound) is a great advantage when compared to similar rated sheet wound units. We have several 69/13 KV 30 MVA units (55 deg C rise) that will consistantly operate with loads of 22 MVA with an ambient temp of 37 deg C with the first stage of forced air cooling not yet necessary. In my opinion, these same units are limited in over-loading capability by the protection applied to them, not so much by thermal constraints. If the range of loading on the proposed unit(s) varies greatly and signigicant time may be spent in the upper end of their rating range, you would be wise to consider very carefully the disc wound unit, even if the price is slighly higher than the sheet wound unit.