They might be construed as one in the same, or they might be construed as switching capacity is bandwidth capacity and forwarding performance is packets per second (PPS). If the latter, you need both. You can bump into a shortage of one before the other. Regardless, your performance will be limited by whichever is insufficient.
For example, in the 3750G, 48 port, its specs note is has a 32 Gbps fabric and a 38.7 Mpps.
48 duplex gig ports need a fabric of 96 Gbps and a minimal size packets, for gig, requires 1.488 Mpps (or about 71.4 Mpps for 48 gig ports). So the 3750G 48 can fall short on both.
BTW, PPS needs decrease as packet size increase. I.e. if the 38.7 Mpps, if still supported for larger packets, it would support much more bandwidth.
By comparison, the original 4948's specs has its fabric bandwidth as 96 Gbps and its PPS as 72 Mpps.
I.e. while a 3750G should be able to support wire-rate for a partial load, the 4948 should be able to support wire-rate for all its ports.
