I've been playing 40k since '98, and have gotten dozens of people into the hobby, there are three things I recommend people take into account when picking their first army, in order of importance.

#1: What the minis look like. A huge portion of the hobby for most people is building and painting, so you really want to go with minis you like the look of. You seem to have narrowed your choices down on this.

#2: Lore. The game changes every few years, but lore is pretty consistent for the most part, read up about your favorite armies and pick the one that speaks to you the most. You may love the look of one army, but if the lore doesn't speak to you that could cause you to lose interest.

#3: Play style: To me this is the least important factor to choosing a first army for a couple of reasons. You don't really know what play style you enjoy, you may think you like static gunlines, but actually enjoy zipping around more. Most armies actually have units that can fit multiple play styles, and your first army will likely be in the 500-1000 point range, which means a couple troop options, an hq, and a special unit or two, and once you play a few games, you can decide to focus on more heavy support, or fast attack, or elite units as you get the feel for what you like, beyond that, the meta changes, one edition may be more heavy weapons focused, one may be more troop focused, a few years back mechanized was very powerful, the foot slogging infantry was the way to go. If you pick an army based on aesthetics and lore, you can (and likely will) expand that army over the years to be able to field a force that has a number of play styles.

The one thing I recommend every new player avoid is meta chasing. Don't pick an army because it's powerful or wins tournaments, because in 3-6 months that will have changed, and you may be left with an army that doesn't interest you.