Bear in mind the fact that if you use your HD300, all the audio processing is being done in the unit and not on your Comps CPU. This free's up tons of processing for proper playback on conversions and running other needed plugins like compressors and eq's. It isnt a big deal if your only doing simple tracking, but if you going full on, you will soon learn how much processing power you need to run 4 guitar plugins (line 6, guitar rig, amplitube etc). Pops and clicks start happening and you can bump into pretty big problems, even with high end machines.
Using the outboard hardware takes a lot of that out of the picture. I recently got a line 6 HD bean, and love it simply for that matter alone. A lot of the time running, say, guitar rig, i would have to record the track with just the guitar direct in, then, route the track output to a separate track. the playback the track with the plugin running and record that on the second track. It was just a giant pain in the ass. But if i ran the plugins on 4 guitar tracks, a bass track or 2, then start adding in the EQ and comps need etc, it would just drop out, and my comp isn't a piece of shit either, it's pretty decent.
This is also just my experience, it may not be yours or others here, just giving you another perspective before you make a move.
