I know this forum is rarely visited...however, I got an answer from one of the sponsor links (whom I also happened to buy these baseballs from)
Posting the response for future reference for anyone else who may be wondering...
In regards to your inquiry, a UV protection of 50% will generally double the lifetime of a signed item before it starts to show fading due to direct sunlight. This can vary depending on the quality of the baseballs which were signed and the ink used, but usually this moves the fade point of the signature to where our grandchildren will start noticing it. Obviously 70% will increase this fading period somewhat, so it'll last several years more than the 50% in direct sunlight. If you keep the item in indirect lighting, such as a darkened room or a room without direct natural lighting, this can increase that time by a large factor. For all intents and purposes, the less natural sunlight that you display an autographed item in the less it will fade and the cases assist in this. The higher the UV protection on the case, the longer this fade time will be extended. So both cases will prolong that protection, one slightly more than the other. if you are intending to keep the!
baseballs for your own collection so as to pass them down, both will work very well. If you are looking to potentially sell these items in the future, we would recommend the 70% case because future collectors look well on additional protection.