As far as the seminars, they are worthwhile. Once. Seems every year is a re-hash of the same ones that you saw the year prior. Some are basically advertisements for guides and lodges. These have titles like "fishing the so and so area". If you plan to go to that place, they're great, but otherwise worth skipping. The better ones are based on techniques and so forth. But once you go one year, there's no need to return every year to hear the same stuff slightly updated.
If you're a fly tyer, it's the bee's knees. Tons of great tiers with desks set up everywhere. You can go watch them, get some pointers, B.S. a bit. A serious fly tyer could spend hours and hours just going from tyer to tyer and talking things over.
As for a shopping trip. Yes, if you have lots of stuff to buy, it's worth it. If you just need some tippet and a shot wheel, skip it. Tons of fly tying materials, cheap flies, and fly shops who sell everything that fly shops sell. All in one place. So if you want something specific, you'll find that someone there has it. Also, if shopping for rods, you can pick a bunch up and go to the casting pools and try them all out back to back. Even if not buying, it's interesting to cast a bunch of different rods and get a feel for their actions.
Selection is best on the first day, and price on the last. A lot of vendors don't want to take stuff home, so whatever they have left, they give crazy prices to. I bought a good pair of Chota wading boots for $10 one year. A few years back I bought a discontinued version of Simms G3's for $300 (retail was $500). At the fly shack you can get like 3 dozen assorted flies for $20. But while the steals come the last day, it's kinda like Big Lots, good if you just keep an eye open for deals, but if you have a specific thing you want to buy it might be gone by then.
And then there's seeing authors and celebrities, get them to sign their books, etc. Which is kinda cool, but not really a sole reason to go.
Somerset is 10x bigger than Lancaster. The seminars and demos are similar. The shopping selection and number of tiers are not. If you go to Lancaster, don't eat there! Go in town somewhere. Somerset has a nice bar.