Short answer: -
Simoun is good,
Hell Girl (Jigoku Shoujo) looked kinda meh and I didn't bother watching all of it.
First thing to mention about Simoun is that it's a shoujo-ai/yuri series (or "girls kissing", to avoid confusing terminology), so if you don't like that, forget about it.
Simoun has an interesting sci-fi/fantasy vibe about it. While superficially the technology is very advanced, the manner in which it operates, as well as the archaic religious system with a seemingly extant deity, give the series a fantastical air to it. This duality is expressed fairly well in the deity, Tempus Spatium ^^
The story in Simoun has two major elements - first the war, largely beyond the control of the core protagonists, yet ever-present, and the second, the relationships between the main characters. As a war story, I think it's done fairly well - both sides have reasons for fighting, neither side having a monopoly on good or bad people, people make good descisions and bad ones. Basically, it has a feeling of realism to it - maybe not as harshly as something like NTHT does, more shocking scenes kept firmly at a distance in Simoun - but if war is portrayed I prefer it not to be just the goodies beating up the baddies, then everyone lives happily ever after. The relationship side of things is also handled well. It's nothing as tempestuous as many dramas, the characters and the relationships between them slowly outlined and developed as the show progresses, so that by the end of the series you're able to understand the choices each one has made.
Other considerations... the artwork was good, effective... the clunky CG much less so, definitely the worst thing about the series for me. I quite liked the OP and ED themes, although I didn't notice anything special about the soundtracks used throughout the series - but then I usually don't.
It may not be something to sit down and watch for some light entertainment, and it's certainly got it's flaws (not least of which is the suspension of disbelief necessary to accept the basic premise), but I'd still recommend it to anyone who watches much anime.
On Hell Girl I've got much less to say (which I'm sure you'll be pleased to hear
).
From what I watched (I dropped it after a handful of episodes) it's a very episodic show, in a monster-of-the-week variety, except that instead of a monster it's someone else getting rather spectacular revenge, then having their soul taken. Doing an episodic show is difficult - while series like Mushishi may be able to make each episode interesting enough to keep people coming back to them, others - like Hell Girl - don't have the same creative spark. I found the episodes unmemorable, with characters whose development was so brief it was hard to sympathise with them.
What I've heard from others who've seen the series in full indicated that while the motivations of people calling on Enma Ai (the hell girl herself) vary, and our red-eyed protagonist gets given some character development, any sort of underlying plot is, while present after a time, somewhat sidelined by new characters seeking revenge.