This is gonna depend on where you hunt and how you hunt, hunting with an RP means you won't rely on your partner entirely blocking for you as the RP can't Exeta res the monsters away, but if you know how to kite then you don't really need to worry about physical damage, it depends on the synergy with your partner so this is entirely up to you and the way you hunt and the kind of hunts you do. Nonetheless if you're hunting a strong melee attack monster with physical damage and you're worried about the impact or getting headshotted then you definitely want more physical protection, you can compensate the 1 missing ml from shoulder plate drinking mastermind potions during the hunts, they give you 10 minutes of buffed ml, but if you're hunting monsters with AoE or strong energy elemental attacks I would use the spirit guide, you can imbue the spirit guide with more shielding.
If the answer you're looking for is which one is better I would go for the shoulder plate as you can also imbue an elemental protection on it to compensate what the spirit guide has to offer and it is cheaper than the spirit guide, but both are great spellbooks/shields and if you can invest in imbues and having them both they're a great adition to your set.