In terms of effect on the beer, not much if anything. In terms of usage, it kinda depends on your system. For one of my kettles, I need to bag whole hops, but pellets can go in loose. For the other one, both need to be bagged. For some people, neither need bagging.
I also need to bag my whole hops as not to clog up in my system, but pellet hops can be boiled loose. I prefer whole hops over pellet hops. Not too sure if there is any significant difference between the two, but I believe whole hops are less oxidized due to processing of pellets.
I prefer whole hops - pellets clog my screen. However, you can expect a higher alpha acid utilization from pellets because the lupulin glands are ruptured in the pelletizing process. Also, if you’re making a big beer with lots of hops and you’re using whole hops, you will lose some liquid as it is absorbed by the flowers. So plan accordingly (i.e. when making a double IPA, plan for 7 gal vs 5)
If you have absolutely guaranteed fresh whole hops, I agree. But in general I find pellets store better and stay fresher longer. I always buy continental hops as pellets.
Unless one knows the hop farmer, it’s hard to do that, but I’ve started growing my own. I don’t get quite enough yield to be self-sufficient yet so I buy from hop direct or fresh hops. I recently (last November) ordered some Centennial and Cascade hops from hops direct that were the freshest I have ever purchased. I mean they were a brighter green color and outstanding aroma. I am very pleased with them.
I use both whole and pellet. I agree with Denny for Continental hops - I prefer pellets for freshness. Not saying you can’t get good leaf hops that are Continental. Recently I have been buying more pellet hops though. Easier storage and very easy to use on my system. Leaf hops also work on my system quite well.
I prefer whole hops. They provide something solid for all the hot break to get stuck to.
Equally important, I have an older (kinda rube goldberg) counter flow system with a 1/4" outlet. Whole hops are screened out upstream, but pellets can make a muck that can clog the line if the flow is too slow.
It sounds bad but works pretty well in that I get good oxygenation during the slow run-out.