Very different devices with different purposes.
Security wise = pix firewall. A 3750 is not a security device and an acl is only a basic level of security.
Speed wise = 3750 but then again depends on how fast your connection is. There will always be some additional latency with a firewall but may not be noticeable.
It really depends on how secure you want the connection to be. If you don't trust the remote users then you should look at the pix. If your remote users are part of your existing company and they are trusted users as such then the pix doesn't really give you anything.
One other factor to bear in mind. If you need NAT then you will need to use the pix as the 3750 does not support NAT.
Edit - just noticed the connection is via wireless. How happy are you that the wireless link is secure ?
Jon