In short, yes there is. The difference between a high capacity LRT and a small subway train is the potential for additional capacity. A 3 car TTC subway trainset and 2 of the LRT vehicles that will be used in TC has roughly the same capacity. However, to date I have not heard anything about the possibility of these trains beings tripled together, so it would be logical to assume that their capacity caps out at being paired. Subway trainsets however, can be pretty much as long as the platforms they serve.
It is unlikely that Eglinton will need anything above a 4 car trainset (like on Sheppard) for the near future. But if there is even the potential for the demand on that line to exceed what LRT can reasonably do, why not leave that door open? You can run 4 car subway trainsets at virtually the same frequency you would run paired LRT trainsets. And if the demand does increase to the point where you need to add extra cars, you don't have to make any modifications to platforms and the like. With an LRT to HRT upgrade, the platform height would need to be changed, and the electrical systems completely redone.
In short, the slightly higher upfront cost of doing grade-separated HRT compared to grade-separated LRT may not seem worth it. But when one considers the costs down the road, should demand come close to exceeding capacity, and an upgrade becomes necessary, spending that little bit extra for the potential of inexpensive extra capacity seems like a good investment. Not to mention it would be technologically compatible with every other grade-separated rapid transit line in Toronto (with the exception of the SRT, the orphan).
Also, doing Eglinton as an HRT line opens up the possibility of something that I have privately considered, and that has been mentioned by a few people a couple times: the possibility of interlining Eglinton with the DRL. It is unlikely that we will see the DRL go north of Eglinton for the foreseeable future (50+ years). So, to eliminate forced transfers at Jane and Science Centre, have 3 routes running along these two lines: 1) Pearson to Kingston (or Kennedy, but hopefully Kingston) via Eglinton, 2) Pearson to Kingston via DRL, 3) DRL Loop (DRL to Jane, across Eglinton to Science Centre, back down the DRL). It may be more complex, but especially in rush hour, I can see it really coming in handy. For people transferring off B-D going downtown, the 3 line thing won't even matter to them, because all trains on that line at that point are going downtown anyway. Just some food for thought.