SMI does basic IP routing (static routes and RIP routing protocol).
EMI does basic and advanced IP routing (OSPF and EIGRP routing protocols; IP multicast routing).
Forwarding rate of IP packets at Layer 3 may be different between SMI and EMI loads, although the hardware is the same. I'll check on this. But I'm pretty sure Layer 2 switching speeds are wire-speed on all ports.
That's about it.
If your network is relatively small and you have VLANs, the SMI should be fine. Each VLAN uses an IP address on the 3750 as it's default gateway; and you can point the 3750's default gateway to the LAN interface of a firewall or router that gets you to the Internet. (On that firewall or router, you're going to have to configure static routes back to the subnets on the remote VLANs of your network, or you can let them learn about the remote subnets from the 3750-SMI using RIP, if they also support RIP.)
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...