SQL Server Disk Configuration Question

SQL Server 2012 running on Server 2012.

Multiple instances on the same SQL Server.

I have enough hard drive configuration to give each instance EITHER:
a) it's own log file hard drive
b) it's own data file hard drive

I cannot give each instance its own log file hard drive AND its own data file hard drive.

So which would be optimal for performance?  Individual data file hard drives (with all log files on a single shared drive), or individual log file hard drives (with all data files on a single shared drive)?

Thanks.