Both devices are great and very similar. iPhone 8 has a glass back and is a bit more costly to repair if broken. I'd go with it to future proof yourself longer since it just came out. iPhone 7 Plus is still a great device. We just bought our son the iPhone 7 for Christmas as it was a great price on Device Payment. iPhone 8 has a newer chip, slightly better battery, more camera improvements for Portrait mode, glass back (mentioned above), wireless battery charging and no printing on the back of the device except iPhone.
Here is iPhone 8 vs. iPhone X comparison but you can change to the iPhone 7 from the drop down for a nice side by side: iPhone - Compare Models - Apple