I would say OBR are more beginner friendly. fewer units to learn, high durability plus healing makes them forgiving, elite army means higher cost per model, but fewer models in the army so still cheaper overall, plus fewer models to paint - and easier to paint too.
Their faction rules, esp the command abilities, are very strong but can be a bit confusing & a lot to keep in mind. more beginner friendly than gravelords, but far from the most beginner friendly in the game. extremely potent - nigh unbeatable - in a straight forward melee scrum, which will likely lead to strong early results against other new players. But they can be very weak to mortal wounds, ranged damage (shooting or spells) able to pick out their support pieces, and especially anything that messes with command abilities.
OBR have tools to circumvent many of these weaknesses, but they're situational - eg strong anti-magic subfaction, strong antishooting command trait, but both do nothing if the opponent isn't running magic/shooting, & have to compete with more broadly applicable alternatives.
obr's overall low speed and low model count also means they can have difficulty playing to mission objectives, while their relatively poor faction stratecies & tactics means they're often at the mercy of the format for scoring points in competitive games. It's sometimes said that in AoS you can wipe your opponent completely off the table and still lose the game due to the opponent being better able to score objectives and battle tactics, and that's more true for OBR than maybe any other faction.
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The gravelords' new book is the stronger of the two overall, with strengths that are harder to circumvent and weaknesses that are harder to identify & exploit. They are more diverse in terms of available builds and play styles, so if you find something doesn't work for you there are other options to try.
They have deployment tricks, deep strikers, fast hammers, fast chaff, hoardy chaff, glass cannons, decent wizards, ok fighty heroes, buff wagons, everything you could want but for shooty units, which means they're better positioned to adapt to future formats. At the same time they've got better tactics & strategies in faction, so they're less dependant on the format for their points.
but the downsides are the inverse of the obr - dozens of units to choose from make it hard to decide what to use & which units work together as a new player. hoardy battleline chaff mean you'll be buying and painting a bunch more models to get to a playable army size, and those models are likely to take a lot longer to paint due to the greater variety of details. More subtle and tricksy faction traits, subfaction traits, and warscroll abilities mean even good units can be misplayed, making it difficult for a new player to tell if they brought the wrong guys or just didn't use them correctly.
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If you're concerned about long term competitive viability, if you have aspirations of keeping up with the ever shifting competitive meta, attending tournaments with intent to actually place well, and want to have an army that can reliably threaten victory regardless of format, mission, or opponent, then the gravelords are probably the better army to pick, but you'll have a harder and slower time getting started.
If you want an army that's cheaper to collect, faster to paint, easier to grasp, & more straight forward to play (at the cost of also being more straight forward to play against), then go with the bonereapers.
Of course, that's all assuming you find them equally compelling in terms of aesthetics & narrative. If one or the other sparks your imagination more, then that's the one you should go with. rules come & go, but models & lore are forever. if you're not familiar with the lore for these yet, check out my 'death faction history' thread here:
Death Factions in the Mortal Realms: a History - Carpe Noctem not yet updated for 3rd edition gravelord & obr books, but it'll give you the general idea of their personality & place in the narrative
