it's a thankless job

First however, a little piece of advice:

The more experienced you are with other classes, the easier you will find playing arty, and the less it will matter which line you pick, as you have (hopefully) learned about things like the maps, situational awareness, the usual deployment patterns and encounter zones, macro-movement (the flow of the battle, basically), etc. A lot of skills you learn when playing other classes are also important for playing arty, but there are some additional skills you will need in order to play arty successfully.

You will need to learn things like how to predict enemy micro-movement (aka the way tanks back up or roll forward at what times), how to lead shots with your long shell flight times, which spots to pre-aim on, the way distance, angles, and terrain affect your shell flight path, when to shoot or when to hold your fire keeping your long reload and aim time in mind, and so on.

But to get to the point, let's take a look at the lines:

I guess which line you should pick depends largely on personal preference, aka on your preferred playstyle. Very generally speaking, the US arties have longer reload and aim time and less accuracy but have more splash damage, while the Germans are more accurate but their splash isn't that great in comparison. While the Americans have very distinct strengths and weaknesses, the Germans' attributes are quite balanced without any extremes.

Also the Americans tend to have a broader traverse arc, while the Germans have very narrow arcs for the most part. (But then on the other hand there's the GW Panther on the German side that features a partial turret with a huge arc. More about that one later, though.) So keep in mind that you will have to turn more frequently with the Germans, which will force you to think more about where you pre-aim.

With the Americans, you will aim and reload forever, and probably miss, but when you do hit, ooohh boy. With the Germans, unless RNG hates you, you can generally assume that you'll hit (as much as you can assume it for an SPG anyway) but you probably won't devastate the enemy tanks as much.

In the end, I guess which line is easier for a beginner depends on the person and their individual skill set.

That was the overview, I guess I should talk about the highlights (in my opinion) of the lines to wrap this up.

With the Germans, it's clearly the GW Panther. Like I said, it has a partial turret, so turning your hull will not be required unless you have to totally switch flank, which means wasting significantly less time on aiming than you would for example in the GW Tiger (P). Unless you use the gold ammo, your splash radius will be pretty pathetic, so never count on it. There will be the occasional oneshot, but again don't count on it. Most of the time, you will do around 300 to 600 damage per shot, of course depending on how heavily armoured your target is. Basically, the GWP is my favourite arty and I really recommend it.

At low tier, I would go for the Sturmpanzer II route. While the other route has better traverse arcs and more range, I think it's good to teach yourself how to play with a narrow traverse and short range first. Also, I love the Sturmpanzer II. The high shell arc, good accuracy, and quick reload make it feel almost like a lower tier Bert to me, tbh.

As for the Americans...Get the M44. Just do it. Trust me. Its HEAT ammo has insane pen, and you can pull off some ridiculous stunts, if you're willing to make some compromises regarding its other stats.

And that's it, basically. I hope I could help! o/