Plane is easier. Get to the gate early and the flight attendants are really helpful. Often you can take baby items on without the weight limit too depending on the airline (baby food, nappy bag etc). I travelled maybe 8 times on a plane with a 5-11month old and they just sleep or play- especially if you’re breastfeeding. Can usually bring 3 checked baby items too without it impacting your weight limit :)
I travelled with my 7 month old from Adelaide to melbourne (even had to detour bc of bushfires) & she literally slept the entire way!!! Super easy and meant we had everything, pram, car seat, pack & play, steriliser for her bottles. Worked out cheaper than flying too. We stopped every two hours to get her out of her seat. Change the nappy & feed.
I've travelled by plane heaps with my son since he was 1, it's much the same at that age as earlier! You might find your bub would sleep the whole time (mine did!) because it's exhausting 😂 we made sure to have plenty of snacks and a few toys to play with that are faves. You can check in 2 or 3 baby items for free, so you can bring a portacot, car seat, pram if needed! I always baby wore so I had my hands free for tickets and bags and such :)