It’s horses for courses but there’s a reason it’s a significantly longer time to command on long haul fleets than short haul. Those that have the choice, and have probably done both, tend to choose long haul.
Fatigue and jet lag might change your personality, but so does job dissatisfaction. The fact that you’re unsure enough to be asking the question probably means you should choose the airline that gives you the most options to suit your lifestyle - which will change as you go through your career.
Personal advice would be BA long haul with a part time bid in on day one - you’re only sacrificing the most highly taxed part of your salary and it’ll probably save your health as much as your friendships and relationships. Even at 75% you’re probably going to be earning more than your non-flying peers and not working nearly as much. The job isn’t half as mentally draining or fatiguing when you know you have that firebreak coming up every few weeks.
You’ll do the same amount of hours as you do now, probably still earn more money and have an abundance of free time spread out evenly across the year when you can use those staff travel perks.
Good luck.