Spells >> No Spells.
Eldritch Scoundrel Rogue >> Horizon Walker Slayer
Those are very different builds, even if both will be able to make use of Dimensional Agility. You're comparing a 3/4 BAB 6-level caster to a full BAB with SLA's; they have one thing in common and otherwise will feel completely different in actual play.
For the Eldritch Scoundrel, probably your best option is to take Dimensional Agility and then take Spell Perfection on Dimension Door so you can Quicken it for free. That means you don't need to worry about the rest of the Dimensional Agility feat chain. This will save you a feat and mean you don't have the range restriction on Dimensional Dervish. Otherwise a lot of basics of the Eldritch Scoundrel build apply; take a two-handed finessable weapon such as elven curve blade or elven branch spear, take vanishing trick and invisible blade for swift action invisibility goodness, and fill yourself with a selection of defensive and utility spells. If you have access to a Goz mask then fog spells will be amazing for you.
For the Slayer, your biggest issue will be that you only start to qualify for Dimensional Agility at the 9th level. Since you need Dimensional Dervish to get your full attack off that's your 9th, 11th, and 13th level feats locked in by this build. If you want Dimensional Savant so you can sneak attack with yourself as the flanking buddy, then that's all your feats up to 15th. It's not a deal-breaker, but it definitely is a steep limitation on your build.
Figure out which build seems more appealing to you, and maybe then you can refine down further on specifics.
You definitely want unchained for this, as you need finesse training for this build to pull together. Before we talk about discretionary options, let's lay out what's necessary:
1 - Quicken Spell
3 - Exotic Weapon Proficiency
4 - Vanishing Trick (Rogue Talent)
5 - Metamagic #2
7 - Metamagic #3
8 - (Rogue Talent)
9 - (Feat)
11 - Dimensional Agility
12 - Invisible Blade (Rogue Talent)
13 - (Feat)
15 - Spell Perfection
So that leaves your 9th and 13th level feat, your 8th level rogue talent, and your two metamagic feats up for consideration. For metamagic I think Extend spell and Silent spell make the most sense; Silent spell works nicely with your stealthy aspect especially when invisible, while Extend spell synergizes with all the utility and buff spells you'll be using. Of course if you've got other metamagic you're interested in feel free to use those instead; Quicken is the only one that really matters.
You do have some choice in which weapon you choose for weapon proficiency. Anything that's two-handed and legal to use with weapon finesse is a possible selection. Elven Curve Blade and Elven Branched Spear are the "standard" choices, but there are others you can pick.
Your remaining feats and rogue talent are a matter of preference. You can pick combat options like Accomplished Sneak Attacker and Weapon Focus, or you can pick something to play more on your utility. Everything you strictly need is already on the build.
You will also have three skill unlocks. Stealth is an obvious choice that synergizes very well with this build. Perception, Bluff, Diplomacy, Disguise, Sense Motive, and Escape Artist are all neat options.
In terms of gear, start with the big 6: a good magic weapon, dex-boosting belt, int-boosting headband, cloak of resistance, amulet of natural armor, and ring of protection. After that, if you have the cash left over a Ring of Wizardry (1st level) synergizes very well with your build. You can spontaneously convert 1st level spells into Quickened Greater Invisibility, so doubling your 1st level spell slots is amazingly good for you. Another good item would be a shadow harimaki; you don't really care about the AC (mage armor is good enough) but the +5 to stealth is nice for whenever you aren't invisible.
For stats, dexterity is your most important stat by far. You generally won't be using magic offensively so you don't really care about your spell DC's, so extra intelligence is mostly for bonus spell slots. Other than that just try to avoid dumping stuff since every stat is at least marginally useful to you (strength is important for carrying capacity, constitution for hit points, wisdom for will saves, and charisma for diplomacy/bluff/use magic device)
You'll probably like long-term buffs that don't cut into your combat actions. Heroism, overland flight, resist energy/protection from energy, heightened awareness for example. Obviously some dimension doors so you can use dimensional agility; that'd likely be most of your 4th level slots. Maybe a couple of scorching rays for when you need a missile weapon, and a few fog-type spells if you get a goz mask or fog-cutting lenses.
I agree with avr's suggestions. Aside from those, Sense Vitals is a good combat buff at these levels, Mirror Image is nice although you must be weary of Truesight and unconventional senses at this level range. Haste is always a nice buff to have. Shield is a good defensive buff at any level. Wall of Force and Emergency Force Sphere are nice option for controlling the pacing of a fight. Since you're an evil party you might consider dabbling with Animate Dead.
Always remember that you are a prepared spellcaster, so don't be afraid to try switching stuff around from day to do to try new things.
Mirror image probably isn't good - when you can get greater invisibility as a swift action it's usually better, and you probably won't know which creatures which can beat one but not the other - but those others do sound good.
Edit: also, see invisibility. No need to let the GM use invisibility against you, right?