Since you've done an accurate wheel rollout measurement the simple bike computer should give you more accurate results for distance and speed than any device that depends on GPS measurements. I still prefer to use my GPS since it gives me a computer record of where I went, and about how much elevation gain, plus it can tell me how to get back or where the nearest decent restaurant (or grocery store, etc.) is located. But for just knowing how fast I'm going or how far I rode any $10 bike computer will give me a more accurate answer.