Well, hmm. I've looked at both locations, and the one by the HEV Boraros ter would be much closer if you were to walk up to the place near Elizabeth Bridge to watch the fireworks, as johnfarago from TripAdvisor advises (he's a really good source of information for Budapest-related travels, by the way). I'd guess it to be about a 30 minute walk along the riverside. Another advantage with this hotel is it's on Raday utca, Budapest's "Restaurant Row." We ate at a different restaurant on this street almost every night we were there - quality goods, for sure.
On the other hand, this hotel is located on the southern end of the sights tourists generally visit (Chain Bridge, Parliament building, House of Terror, etc.) and in that regard, it's not as ideally located as the other hotel option. But again, Budapest is walkable and outside of the St. Stephen's Day fireworks, public transit would come in handy.
The Maimason Hotel is a longer walk from the Elizabeth Bridge, likely about 45 minutes. But it's not far from City Park, Heroes Square, the House of Terror, and the Opera House, among others. And it looks as though you'd be well-situated to eyeball some of the daytime St. Stephen's Day festivities (state ceremonies, marching bands, etc).
It's a tough call, as you can see they both have pros and cons but if you were more concerned about being near(er) to the fireworks, I'd stay at the southern location. For the daytime activities, I'd stay at the Maimason.
Lastly, I strongly urge you to stop by the Great Market Hall while you're there and head up to the second floor where you'll find the langos stand. I highly recommend trying one while you're there if you aren't averse to deep-fried goodness covered in sour cream, fresh garlic, and topped with a sprinkling of cheese, for good measure.
Good luck!
