Depends on what you will use it for. I will write u here a brief (or not so brief) balancing out of each.
Solar engine
- First: it is magical. U need to spend channeling dice to cast this spell, and as it needs a 3 or more, to play safe (ignoring dispelling) you would need to roll two dice (not very likely to get a double 1).That means that if you play it along many wizards it might loose effectivity as you will spend the power dice on spells, and therefore the bastiladon will just be a chunk of meat in the middle of the battlefield.
- The actual impacts. The number of impacts and strenght of the shot might vary depending on what you roll. I´d say the average you can get on dice rolls is usually 3s or 4s, so what you should expect is about 5 shots of strength 5. That is very effective on mostly high armored troops (Black Orks for example) or monstrous infantry (Ogres), and, of course, characters outside units.
- Side effects: +1 initiative on units within 6 inches of the bastiladon. That is always a plus. And also the -1 of BS and WS on impacted units from a roll of 6
The ark of sotek
- Jungle swarms. If your army includes any number of units of jungle swarms, this is such a HUGE plus. On a 4+ you get to add another base (That means on average one each two turns). This is 5 more wounds and 5 more attacks each two turns to the jungle swarm unit, enough to keep it alive for many turns and maybe for the whole battle.
- The ark is both for close combat and for distance. First: All enemy units within 1D6 (min average 3") receive 2D6 (average 6) impacts of strength 2. This is great for "filler troops" like skins. Infantry of small base. I find it extremely useful against blocks of zombies: kill about 10 (ark, bastiladon and riders) and unstable rule kills as many as points you have won the match by (if you charged and killed 10, and the zombies are 40 in 10 by 4 arrangement (and they don't usually inflict any wound) --> 11:3 --> you won by eight, so overall you killed 18) they flee, pursue, hopefully catch them and bye 40 zombies
So solar engine for "big" troops and characters, and ark for block units. That´s the whole of it. I have tried both and they work pretty well. So now you decide which one you want to build, depends on the armies you usually play against and your own playing style, I guess. But anyways, magnetizing them is definitely the best option.
Hope that helps
Jorgik