Hi Dan,
DI API is not recommended for use in web application (slow, unreliable).
Always measure, but I think your main problem with be the licensing scheme (not really adequate for Web applications).
DI Server is being phased out, and there is no HANA version.
According to your need it could be adequate, until you find it necessary to migrate to say the Service Layer or B1if.
The Service Layer is only available in the HANA version.
Yep, your statement says it all, but if in the future your target audience is in HANA this would be the way to go.
B1i is only suitable for integration and not intended for my use.
I heard that you can create web services using B1i that could be used by your web app. (In any case you should check this out: Creating Web Services with B1if and Consuming b... | SCN). There are youtube videos as well.
In any case my main advice is that, it's clear that you'll have changes in the future regarding the backend of your application so one great advantage you can give yourself is to decouple your application/frontend/domain from your backend, and when the time comes swap the backend implementation for a more adequate one.
Cheers.
Best regards,
Pedro Magueija