I think the question you want to ask is, "What is the difference between using a layer 2 vs. layer 3 switch to connect two floors and 170 users?". Spanning tree protocol is designed to close off forwarding loops and prevent broadcast storms. It will work the same on either type of switch.

In short, a layer 2 switch forwards packets based on MAC address only. A layer 3 switch can forward on IP address and provides additional features for filtering and QOS.

As for the effect on DHCP, that would depend upon your connectivity needs and your addressing scheme. All 170 users could be part of the same DHCP scope and use the same default gateway. You could also create vlans on the switches to isolate work groups and assign separate DHCPscopes to each. That works well for in-group communication, but communication between vlans requires a router. I hope this has helped clear things up a little. Write if you've got more questions.