I'm fairly certain all Z's had hardened valve seats...no?
First off, take a look at both engines and tell us what casting numbers are on the head. (between #1 & #2 sparkplug) Should be E31, E88, N42, etc.
This will sortof determine what way to go.
On one hand you can have good fuel economy, decent power, and EFI cold starts.
On the other hand you can have simplicity, a bit less economy, a bit more power and a choke.
In the end, you can't go wrong with either setup really. If you are planning on driving the car a lot, I would say EFI should be chosen. If its a weekend fun car, the SU's are just fine for that.