I use HSSE, but iI think both instruments are the same with regard to general panning and the interval vs. on the board mixing question. This applies to panning and any other similar production method using Halion or, really, any VSTi.
Here’s how I see it. Using the instrument’s mixing panel means you are essentially using the MIDI Faders, rather than keeping them “off” and invisible on the Mixing Desk. So, it can add to fader clutter, or it can make things more pre-arranged and ready to be introduced into your project. Is it clutter or is it greater precision and detail? You have to decide. If you pan, and the MIDI channel is assigned to Master Output, the Pan effects anything assigned to Master Out.
I don’t think one method is better or worse, but using the Instrument’s Mixer can hide what’s happening if you forget, for example, that Channel Two is panned Left in the instrument’s mixer, and then, in the mix, don’t see any panning on the audio Output to which Midi channel Two is assigned. “Why is that Trumpet panned so far left? Oh, yeah, the internal mixer pan.”
If the instrument is playing a Multi, one sound with multiple voices, and all voices are routed to Master Out, then, the internal mixer can be useful for creating that entire “voice.” Automating the internal effects and the preassigned Quick Controls can make using such “Multi” easier or neater.
If you’re using the instrument in a multi-timbral fashion – five piece band on five stereo audio output channels and any number of MIDI tracks; string quartet; brass section – those can be mixed and placed within the instrument and the outputs all sent the Individual Outputs, or that entire “Band” or “section” can be mixed within the instrument and then sent to Master Out. As far as panning, effects and so on, I think both are great and work in a complementary way.
It’s also sometimes useful to make Master Out the Master Effects channel, with the other MIDI Channels routed to Individual Audio Outputs and the Master Contains only the Effects from, say, a big voice Multi patch of some kind. Of course the instrument’s Aux effects can be routed to any audio output.
I think both, or all these methods work and there’s no right or wrong way to do it. It’s all about the Ray Charles Method, which is, “yeah, but how does it sound, baby?” Which, as I understand the story, was Ray’s answer to an engineer or producer extolling the use of some new technology.