Just wondering what the differences are with many choices.

Here are a few examples

All Shimano must be 12-25 gear ratio

CS HG - 50
CS HG - 400
CS - 6500
If someone could tell me the difference, I'd be greatly appreciative.
Thank you

The more you pay the better the steel and the shinier the finish? Possibly more attention paid to the lightening cutouts too - with better steel you don't need so much for strength (but the rigidity is the same as the cheap stuff).

All three will have the same shifting performance and durability (maybe slightly less for the 400 without the nickel plating) but I believe the higher the price, the lighter the weight and the HG-50 and 6500 will have a more polished nickel finish. The 6500 uses an aluminum spider for the larger cogs making the whole cassette lighter. I don't know the exact weights of each but if a few grams are not important or having shiny cogs, just go with the HG-400.

superior alloys, better manufacturing techniques, advanced shift ramp/tooth form technology, improved finishing methods, lighter weight, and attention to details.

Ah, I meant to check for an aluminium spider, but I was distracted by life.

Thank you for all the information

Really appreciate it.

Thank you

I'll order the HG 50 or 6500

The HG-50's are inexpensive enough, you can get a couple different sizes.
A dozen years ago when I'd do a LOT more miles, I'd typically use a 12-23 in Summer & switch to a 13-25 in Winter when the motor wasn't quite as good.
On my Ebike, I use a cobbled together 14-30 from HG-50 parts.

This past year, I haven't looked at levels, just gears. Why are 9 speed cassettes so hard to find?

I mean I found 46 different cassettes in 16 models for 9 speed on one of the most popular distributors for bike parts in the U.S. Certainly not as many as back in the late 90s early 2000s when 9 speed was new and still really high quality stuff but enough to keep my two current 9 speed bikes going without a problem and without resorting to Tourney level crap. I will note some of that is Linkglide compatible stuff but there is enough to keep rolling just fine and probably a good deal of NOS stuff if you need a super high end set up.

I think a big part of less parts is the pandemic hit everyone hard so making 10 different levels of old stuff that is not common anymore doesn't make as much sense for many of these manufacturers who are trying to save some money and cut down and some SKUs so they can keep in profitability as they recover from the pandemic which seems like so long ago but it wasn't a light switch but more like a big complicated turn on procedure that takes a really long time to get going fully.

Shimano still lists 11-32 11-34 11-36 because clearly 9s road bikes don't exist.
Have some sympathy for people who are still on seven speed, which has replaced six speed as the rock-bottom sub-entry-level quality. SRAM lists only a 12-32 (apart from their "gravity" 7s) Shimano has 11-28 12-28 12-32. Microshift has 12-28, SunRace lists 12-24 11-28 12-28 - the only 24t low out of the whole lot. What do I do about replacing my 12-21 7s? Even now when there's 7s stock still in the system, I'm not seeing any 12-21t from regular suppliers, just used or overpriced stuff on eBay.