I was with Ernesto from 1983 to 1993 (he ranked me 5th Dan) and with the Professor from 1994 until his death.
So my knowledge about Ernesto may be a little old, but I saw an exhibition from him about a 2 years ago and it was pretty much the same stuff I have learned.
But coming to the differences:
Ernesto put more emphasis on form and posture, having a proper stance and an upright position. A little more Karate like but not so stiff. Remy really never lost a word about stances. Anyway, one of the main differences I found was, that Ernesto concentrated much more on hitting, butting, striking and Abanicos and also disarmings, where Remy put much more emphasis on controlling, locking takedowns, pins and trappings. Everybody who ever was in his "fangs" knows what I am talking about. Thats why I am really happy to have learned under both. I got "the best of 2 worlds".
Ernesto teaches a little more vartiety of weapons, also Bo (long staff), or palmstick, Sai, Tonfa etc if you want (even Kendo) where Remy concentrated more on the singlestick and empty hands. Of course he also knew the other stuff and occasionally taught it, but when ha taught me, he stayed more within these areas. In the beginning of the 80ies Ernesto had only 3 Sinawalis (Single, Double and Reverse) and almost no drills. Neither empty hands, nor knife. Exept of his "Freestyle sparring" which is a kind of basic Tapi-Tapi. This seemed to have changed, so that he teaches a lot of back and forth drills that you use with interchanging weapons. Where as in a lot of Remys drills, there is a dominating, controlling person and not 2 people playingthe same game.
Ernesto differenciated also more between "Modern Arnis" and "classical Arnis".
A thing the we still do and it gives us sometimes problems, because Remy did not the same way (at least when he taught us).
The difference was, that in the classical styles like Hirada Batanguenia, Palis Palis, Figuer 8 etc, you could inerchange the stick for a machete. In Modern Arnis you could not, because in modern we try to catch (trap) the STICK of the opponent. Something I would never try with a sword. Modern had the modern approach, that in Germany or in the cities of the US, not everybody is wielding a large blade, therfore other techniques (like the catching) could be used. And the classical styles were the roots of the Modern Arnis, because in the Philippined the DO run around with their machetes, at least in the provinces.
One of the biggest differences though is, that Remy was lefthanded and , at least in the late 90ies, he handed this skill onto his students, in his Tapi-Tapi. I don´t say that Ernesto does not have left handed techniques (his son Jan-Jan is left handed and supposed to be really good) but he did not teach to everybody. Remy made all of his students do the right and the left hand in his Tapi-Tapi.
On a very personal note I think, that Remys Modern Arnis was a lot deeper, what really showed in the comparason of the Freestyle of Ernesto (which was very special to him and one should not show this to lower belts, because it was the "secret of the art"). But compared with the amount of techniques, their depth and how intricate and sophisticated they are in the Tapi Tapi, the Freestyle now seems quite shallow to me.
One last word. Remy was not only a Grandmaster, to me he was a Gand Master, also on a peronal level. He was caring and tried to make you learn and make you good. He was very sure aboutm himself and his position. I did not have this impression at all with Ernesto.
I hope that helped.
But please keep in mind, that this is a very personal view and the others might have had different experiences with both ot the Presas brothers.
Regards from Germany
Dieter Knüttel
Datu of Modern Arnis