I think that you should concentrate more on the security side instead of performance. The stacking technology in my opinion gives you things like -> 37GBps backbone, single management view of the stack.
I have not configured the stack on a 3750 yet but I guess that this interface would be a trunk interface. Further more, you'll need high bandwidth backbone for things like VLANs routing. In your diagram, I don't think that you'll want VLANs routing but instead going through the two firewalls. I would recommend you to do the Separate configuration for security reason.
When you stack the 3750 using the Stack port at the back you have the 32Gbps common backplane. It really doesnot matter the kind of connection that you have over the WAN.
An Ideal design would be to use the option 2 i.e separate switch both for WAn and LAN. In this option you have total control over the traffic flow from inside to outside, which enhances the overall security of the network and thus making the troubleshooting, access control over the traffic flow on the network.
This option is also lot simpler to implement and easy to work out in problems.Whenever you are desiging the network the overall security should also be the main consideration.
Stacked technology is usually used in Server Farms Scenarios. Where you have a server farm and you need High availability and through put to this server farm.
Stacking the switches will provide a high throughput backplane and single management through one switch. THe stack cables are not configureable and once you connect them you don't need a trunk or anything else. This is because the stack cables have their own stack controllers at the back of the switches.
As for your scenario, as said before you have to care for security more than throughput. This is because your WAN is not more than 10Mbps. However, Scernario two you lose the High availability option because your servers will be connected to one switch. New servers have the teaming option where you can connect each interface to a switch and they worke in a failover or load balanced fashion thus improving the availability incase one switch goes down.
If I was in your shoes, I would buy a small switch for the outside zone because we will not need a L3 routing or lots of VLANs and we have small bandwidth 10MBps. A L2 2960 or 2950 switch would be sufficient.
As for the 3750 I would stack them and use them on the inside and I won't lose their high availability option or their high throughput. Also, you will be secure coz you physically segement your network.
IN short, if you don't need high throughput and L3 routing on the outside zone then using 3750 for this the outside is not suitable.
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...