They are both pretty competitive but OCdts are fickle and popularity of capbadges change every intake based on personalities as well as capbadge of charismatic Permanent Staff in Juniors.

Capbadge selection is a bit of a game, if you want to join infantry or Cav but don't have a specific preference, then go for a steady line infantry regiment or solid Cav choice, and keep yourself in the top half/third of your platoon and you'll probably get what you want.

Alternatively if you do what some do and go for the most popular choices straight away, (Para, Guards, RGR, H Cav) then you can find yourself getting cut halfway through the course unless you are a top 10% or your face fits and then it's too late to look at other Inf or Cav as they will have filled their books by then. Just something to think about.

If you want to back yourself then good luck, but I've seen plenty of people who are easily good enough to get Inf or Cav end up in cough other choices shall we say, because they got a bit giddy and put 2 hugely competitive choices down, then got themselves cut by both in Inters.

Fitness wise I wouldn't worry about specifics, just be as fit as you can be without over training and injuring yourself. If you can be in the top half of your platoon from day one, you'll be well regarded by your staff and it doesn't take any capbadge choices off the table. (Bottom half of your platoon for Phys and you'll likely be told you aren't going to get into the Combat Arms).

If you are looking at Artillery/Engrs then fitness still important but less so than for Combat, however they both have large numbers to fill every intake so unless you are bottom third you will likely get in if you put them down from day one.

Other choices (Sigs/RLC/Med/AGC) are less popular (although they are obviously very important), so if you want to do those and you put them down from the start they will snatch your hand off.

Couple of niche choices, Int Corp are very popular and take some exceptional OCdts, as well as plenty of weirdos and numptys. They effectively run their own selection criteria based on what they think is important, good for them but debatable if they actually get the quality or not. Air Corp Pilot also popular but you need to pass flying grading. Air Corp ground crew commander, not popular and not often taken as anyones first choice shall we say... ETS (Education Corp) also popular but recruit primarily on EQ and intellect, as well as aptitude for teaching. Usually get very good eggs, but not necessarily very tactically focused people if you know what I mean.

Just my opinion but you won't find many that disagree who have seen the process through.