Which do you recommend, Cisco ASA or Microsoft ISA Server for firewall solutions?

I think Cisco is good choice

 I wonder why people (especially IT guys) compare Hardware Firewall with Software Firewall! Ideally software firewall must be used in conjunction with software Firewall. It is obvious, for packet filtering only Hardware Firewalls are always superior but perform only filtering and NAT function whereas Software Firewall like ISA Server perform many other functions other than filtering like publishing, proxy etc. However, if someone budget do allow then obviously Checkpoint and CISCO ASA are market leader in Hardware Firewall.

ISA server basically used for internet trafic control, but it can also use as firewall because you can configure firewall rules in it, but choice is depand upon your company policy and your IT budget.

but also note that hard firewalls are prefered as compare to soft firewalls.

You should ask this question before Microsoft quit the line of producing firewalls. So the answer now is, of course Cisco ASA. :)