For Bio: actual DAT is harder than kaplan because of many random Qs.
For GC: kaplan is harder, but it kinda stressed too much on calc. whereas the the actual is more conceptual.
For OC: kaplan is harder, but not by too much.
For RC: I didn't take the kaplan subject tests for RC...
For QR: kaplan is wayyyyyy harder, but doesn't have enough trigs.
For PAT: kaplan is considerably easier than the actual.
Hope it helps, just my 2 cents.