First of all, welcome to TFL!
Two pieces of advice are in order: 1) you'll often find that folks at gun shops don't always know what they are talking about. A blanket statement like that is irresponsible.
2) if you don't have one or two reloading manuals, you need to get them before you can think about any serious reloading.
IMR4350 doesn't require a magnum primer so a standard one is what you need. It isn't a question of which primer type is better, they are designed to ignite different types of powders. Think of it as similar to octane in gasoline. Some engines require a higher octane (higher compression needs a slower burning gasoline) and some require a lower octane. Neither type of modern gas is "better" than the other. (No discussions of additives is necessary - it's an analogy.
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