2nd parallel card or Mesa?

For you it is easier with parallel.
In general, depending on the mesa card, it is about the same for normal configs, with the added benefit of having more options with Mesa cards and much higher speed.
Parallel can do 20000 pulses per second if you have a very good computer ( not new or powerful, just good latency ), Mesa cards can do over 1000000 pulses per second and so not need low latency computers.

The G540 had one poor design decision that makes it unusable for some use cases, not all.
The divider by 10 on a controller that works from parallel port that has limited capabilities in producing steps !
In most machines that use reduction and requre higher motor speeds, the error caused by the divider is not noticeable much, but the overall speed is limited a lot. Driving a 20tooth T2 pulley directly from the motor will show that error a lot.
Overall they are nice drive and the electronics inside are very good.
Had a G540 brought in for service as it had 220V put through one of the outputs and did a bit of smoke magic, it was still working properly, it is still working for over 5 years now, just the outputs did not work.