They're 480p.
Honestly I found Gradius Rebirth and Contra R to be both bad games, between their overreliance on boss and set pieces from the previous games (The first boss of Gradius Rebirth is Big Core, a boss so overused and simplistic that he was made a midboss in every following gradius games besise Nemesis on the game boy, the final boss of Contra Rebirth is the same as Contra 3's but sometimes a bunch of enemies appear from the left of the screen...), art that range from mediocre (Gradius) to awful (Contra), gameplay that simplify or ditches good features from previous sequels for no good reason, ... etc. I could write a novel on how much I dislike these games.
Now to be fair, I don't have everything about the Rebirth games. I like how the 3 loops in Gradius Rebirth have different level layouts and how the difficulties in Contra R sometimes significantly change enemy placement and the properties off boss attack. I also heard Castelvania Rebirth is notably better, but not being a fan of the underlying property and on account of being so badly burned on the other two Rebirth games, I have little incentive try it out.