Good info!
Well, to start, you never want to deploy a critical service on hardware you are suspect of. If the service you need is mission critical, it should be on hardware you can rely upon and trust.
How many Windows clients are you looking to support? Tiger Server can provide domain login for Windows clients in as a PDC and provide NT style domain support. Note, Tiger cannot be an AD controller or a PDC/BDC as part of a Windows installation.
Of course, Open Directory will be able to provide login support to your Mac OS X clients, in addition to managed client settings. You could also look at something like pGINA on your Windows clients to allow them to directly talk to the Open Directory as an LDAP client, as opposed to using the built in PDC.
You do have lots of options, but if you want to provide platform specific management, you may want to look at using an AD-OD installation, if you feel you can maintain the AD itself. The OD option has the key perks of better hardware and a newer software platform.
From what you have seen in your own environment, how do you feel you should proceed? There are always lots of variable that you'll see daily that anyone on here won't, so your inclination may be correct.
Michael