ECG electric design: 1 and 3 op-amp designs from a safety point of view
I am not sure of what topology you are referring to or if this is an “ideal” type of situation, but I believe this might be irrelevant.

In reality, both topologies are equivalent from a safety point of view. You have to explicitly introduce isolation barriers when it comes to medical applications. 

Although op-amp and instrumentation amps have very high input impedances, these impedances are only valid if maximum ratings are not exceeded. You will always have a conduction path through their protection circuitry. 

It might be possible to find instrumentation amplifiers that can tolerate relatively high common-mode voltages above their supply rails, but that just means higher maximum ratings. 

In medical applications you have to design for the possibility of circuit failure (and in some cases the presence of defibrillation voltages), this includes a worst-case failure of the amplifier itself.