I've actually lived temporarily in both, and although my views may be distorted by the reasons I was in each place I'd chose Lancaster every time.
Admittedly it was twenty years ago, but Brighton for me was near-London prices without London incomes, some very pretentious people, a lot of bullshit as you point out, big enough to be impersonal but too small to have the big-city distractions. And when I was there the city centre had a monumental garbage problem because the refuse collection consisted of putting black bags out on the pavement, which the seagulls attacked. I rented in Preston Park and commuted into the centre, but I found living there a grind.
It's also fine if you want to fly from Gatwick, but less so for other south eastern airports.
Lancaster has the look and feel of a large provincial town and is under no illusions about what it is. Lots of independent shops, great food scene, and I found the people simply much more genuine and friendly. It's also under an hour from not one, but two national parks. If you're set on beach access that's in Morecambe, which is not the nicest place on Earth or even in the North West. The town centre has been prone to flooding, so have a think about this when choosing where to live or buy, and it also has a hideous one way system which backs up badly at rush hour.
Lancaster is counterintuitively warmer in winter because it's wet and doesn't get affected by continental weather systems like Brighton does. It's also much less windy. Summers are hit and miss, usually miss.
The only other thing I'd point out is that whilst it is feasible to live in Lancaster without a car, going out of town without one is not for the faint hearted and you will want to get out for a variety of reasons (not least because the Lune valley has some incredible pubs!). For non-drivers Brighton is probably the more sensible.
You'll know what's best for your career, but I was in Brighton to work in marketing and found it has a couple of good creative agencies and something of a 'scene' for it. Never had to look in Lancaster, but I'm not really aware of anything similar up there.