I am not a cardiologist-just an engineer who has slept in a Holiday Inn.
Having said that, I'd be surprised if the medication made a signficant difference between the two situations in terms of overall benefit from cardio improvements from exercising at the 70% level. I could some impact if the beta blockers prohibited allowing the heart rate to get above a 70% level. I would think that the person without the Beta Blocker might be able to have a higher power output since he could probably raise his HR higher-but in terms of the benefits of exercise, both situations should lead to excellent improvements in both strength and efficiency if the exercise is done with reasonableness.
However one area that could make a difference is with the "interval" training-where the HR is raised to above 80% for a period and then allowed to drop back to 60-70%. The non-beta blocker candidate might be able to see a little more aerobic improvement over time due to those benefits derived from that training.
A friend of mine went on Beta Blockers and found his "top end" and overall output was decreased after going on it. He was not satisfied with being in that situation and has changed medications.