Virtualizing Oracle : better use dual or quadri processor VBlocks?

Yes, the answer would be "it depends". They may want to provide a mixed model where one can choose small/medium etc. In addition to the core/memory requirement, having some estimate on how many total Oracle instances they are projecting would help also.

If you still need to come up with a guess, than here is what I would do.

Look at the blades they are using and make sure that all the VMs fall within NUMA configurations. The most popular blades are UCS B200 M3 with 4 core or 8 core sockets (8 or 16 cores per blade respectively) for Oracle. Next determine the memory required for each VMs and come up with small or large configuration. For example, you can start with  2 cores with 16GB (small), 4 cores with 32 GB (medium) or 8 cores with 64GB (large). For production, I usually keep 1:1 physical to virtual ratio.

So if they are using B200 M3 E5-2643 (4 cores/socket) with 96 GB memory, than I would pick the 2 core model (if you had to pick just one)  so that they can have about 4 Oracle instances per blade. These are probably the best processors in terms of performance. The B200 M3 E5-2680 (8 sockets per core) would give them about 8 x 2 core Oracle instances. The 2643 are faster processors than 2680 processors, just fyi.

I would definitely strongly consider the 2 core blades. Remember that Oracle licenses their product on the core basis, and the customer must pay a license on all cores which were installed on the box at the factory. That means that you cannot disable CPU cores to save license costs. Many Oracle workloads are not CPU intense, but are rather bottlenecked on the storage, memory, or some other resource. Having idle CPU around is spectacularly wasteful in an Oracle context. Make sure you design your system with the minimal core count required.