I respect your guys opinions on the date of hire thing but...
Integrations should be done with mininal impact on the people being integrated! If you are in the middle of the pack at your current company then you have a certain expectations with regards to schedule, quality of life, upgrade and whatever other good things come with your level of seniority.
Date of hire will take away from the smaller company being integrated by putting people lower in seniority at the integrated company taking away from them what they had at their company before being integrated.
Relative seniority is better because no one looses anything. Everyone is at the seniority level they were at before integration. The impact of the integration is minimal. All that leaves is people who get upset when someone else appears to be getting ahead faster then them. That's like getting upset that some people upgrade in 3 years at Colgan when it may take 6 years at Pinnacle. The price for that 3 year upgrade was working for Colgan.