I would go 1/8" to 1/8". The 1/8" we're talking about (on the D50, R09, etc) is unbalanced by definition, so you are not having balanced going into them, even if you use an XLR output (since you'd be unbalancing that output to use it).
There are a number of ways to properly unbalance a balanced signal depending on the equipment involved, so if you choose to unbalance the XLR outputs, you might do it improperly. Plus, the PSP2 will need additional electronics in the signal path to create the balanced output after the gain stage. Presumably the unbalanced output doesn't go thru these electronics (ie, it is straight to unbalanced out, not unbalanced post gain stage > balanced > unbalanced out). So from a sound standpoint, I'd say 1/8 to 1/8 would be better.