Bottom Bunk Bed:
I walked from Mid May to end of June in 2018. I usually arrived between 2:30 and 3:30pm. I got a bottom bunk about half the time. As mentioned above if having a bottom bunk is a priority the earlier you arrive each day the better your chances along with the earlier in the season you walk the Camino and in April, I'd guess if you arrive by 2:30pm you will have a bottom bunk 90% of the time.
Benefits of Bottom Bunk:
Physical - Your weight, flexibility, mobility, strength. Getting in and out of the top bunk can be a challenge to execute gracefully. If you need to use the bathroom during sleeping hours you will need to get out and then back into your bunk, in the dark, each time you need to pee.
Organization - you have a "large", semi-private useable space for all your stuff when you get the bottom bunk. The space under your bed is a great closet for stuffing your shoes, poles, back pack and clothes you will get into when you wake up. You can hang a clothesline above you on the bottom side of the top bunk to dry out any wet laundry overnight or your towel after a shower. You can hang your glasses and your head lamp from the bottom of the top bunk so you don't crush them at night and you have your light within reach. But most important you have both a "chair" and a "large desk" which is important. You can sit on the lower bunk as a chair to rest for a moment, to put your shoes on or take off...to dress and undress. And to sit with feet on the floor while you use the rest of the lower bunk as a desk for organizing your stuff. I had organization challenges, especially the first half of my camino so having a bottom bunk reduced my stress considerable. I never packed the night before and packing in the morning from a top bunk is impossible if your bottom bunk mate has not left. When in this situation I would not leave bed until the lower occupant had vacated. I'd then use their bottom bunk to work from, organizing my getting dressed, packing and departure routine. I frequently had some level of friendship or familiarity with my top bunk mate and always invited them to feel comfortable sitting on my lower bunk when it would make their life easier while I was awake or away from my bed. It is rare for there to be chairs or benches to sit on in the bunk rooms so if you have a top bunk you may not have any place to sit other than outside of the bunk room or on the floor.
Fear of heights or rolling out of bed - someone did roll out of the top bunk at the big Albergue in Roncevalles and got hurt. It was on the other side of the wall from me so I did not get the details but it sounded somewhat serious. I think 50% or more of the bunks had some kind of protection discouraging a night time roll out which is good but those things also interfere with getting out of bed at night making a mis-step while getting to the ladder more likely.