Span will capture everything but is simple
VACLs allow granular control to certain types of traffic can be filtered out by only using capture on the traffic that you are interested in. Care should be taken with your filters otherwise you could be ignoring genunine attacks.
It is better to run IDS on span for some weeks prior to tuning your VACLs to establish what you what to capture and what to ignore. This applies even if using span ports.
IDS does require tuning but one that has been done, it works really well with either technique.
