Whatever you do, don't watch the Lock Picking Lawyer's videos on youtube. He will make you absolutely paranoid that nothing is good enough.
You need to realistically determine what your needs are going to be. Are you just stopping in for coffee? Will you be out of sight of your bike for minutes? Hours? Overnight or longer? Are you going to be parking in the same area regularly enough that someone may discern a pattern and target you?
I went overkill and bought that 5 foot fuggettaboutit kryptonite chain and lock. I carried it once. Way, way too heavy to take with you. That's the setup to use at home or at work where you can leave the chain locked to whatever you're gonna lock your bike to.
For going somewhere for lunch or meeting with friends for a couple of hours I bought 3 feet of laclede 10mm through hardened security chain for $25 or so and 3 feet of pewag 7mm security chain and a viro lock for about $80 from westechrigging. I use the kryptonite ulock with the laclede chain through the frame and rear triangle and the 7mm chain for the front wheel. I think it's a good compromise of weight and security. It's heavy, but less so than the kryptonite setup I had. If I know I'm going to have my bike in sight I take just the 7mm chain and leave the ulock and 10mm at home.
Of course anything that is removeable goes with me, including the seat.
Steve