Difficulty of work from hardest to easiest: Frac>Wireline>Vacuum Truck>Crude.
Number of hours per week will vary depending on who you hire on with and where. Frac crews work very long days, often only getting a few hours sleep per day. The hourly rate for frac tends to be lower than the others but they more than make up for it by working 80-100 hours per week. Wireline will involve similar hours when they're on a frac, but they do lots of work on the production side that involves somewhat lighter days. Crude haulers tend to work 12-14 hour days. Driving a vacuum truck with the right company will get you 12-14 hours per day. A note about vacuum trucks: some companies pay by truck time versus ticket time, i.e. they bill 4 hours per lease haul regardless of how long it takes. Therefore if you can complete the haul in 2.5 hours, you still get paid for 4 hours. I know some drivers that bill more than 20 hours per day and are DOT legal. These tend to be small companies that undercut the bigger ones and pay the drivers more. It's great for the drivers during boom times, but once things slow down these companies will be the first to disappear because they lack the customer base of the larger companies like Nabors, Basic, and Key.
IMO vacuum truck is more difficult than crude because you will occasionally have to clean up spills, wash out pits, etc. But it's not hard work by any means.
As for stress, I guess that depends on your personality. I'm sure frac can be stressful due to the lack of sleep. Wireline deals with explosives so there's always that.