Here is my thief build. You will see that there is a lot of overlap with my assassin build. To be sure there's a lot in common between the two play styles in game mechanics. This is where roleplaying comes in. Many strategy guides recommend stealth characters should join both the Thieves Guild and the Dark Brotherhood. Astrid and Delvin Mallory are old friends which further reinforces how well the guilds mesh. I enjoy the game more when I limit my characters to no more than two factional quest lines. My barbarian did the Companions and the Stormcloaks. My mage did the College of Winterhold and the Imperials. My stealth guy did the TG and DB. Sometimes it's fun to do one of the stealth quest lines and not the other. For example, the TG never kills its marks. Your thief could take a similar stance and suddenly have pangs of conscience when about to visit Aventus Aretino. Even without the Shrouded Gloves, the Assassin's Blade perk will allow you to one shot many of the trash enemies, especially if you pickpocket their armor off before stabbing them in the back.
You want to be careful about Pickpocket. It's one of the fastest leveling skills in the game, especially if the Thief Stone is active, and the bad guys will scale beyond your combat skills if you're not careful with it. When you think about it though, that squares with the thief's lifestyle. He avoids combat unless absolutely necessary. And with all of the TG fences you unlock throughout the quest line, you'll easily become rich enough to just pay for training in One Handed and Archery (and then steal your money back, lzzozozolozozzolozlol). That's how I roleplayed my guy: he's had the best training money can buy but where the metal hits the meat, he prefers to sneak by as his first resort.
Many players have said they disliked the TG quest line's turn toward mysticism near the end. Personally, I think the Nightingales are awesome. Their armor is my favorite in the game. It's silly that the mask conforms to a human shaped head even if your character is Khajiit or Argonian. Bethesda can make armor have boob plates when worn by female characters, but they can't make a cowl conform to a snouty guy? As per usual, some races - Khajiit and Wood Elves - are the "best" at being thieves according to their starting skill bonuses, but any race can make a good thief.
As always, there are enough perks left over in my build to branch out or invest more before you hit the soft level cap of fifty. The only truly useless skill tree is Lockpicking, particularly if you play on a console like me. Lockpicks are cheap and TG fences always have them for sale. I've become good enough at the minigame that I can pick Master locks after five or six tries. I eventually put my spare perks into One Handed, Light Armor, and Archery. You'll look like an outrageous badass stealth rolling across the landscape in your Batman outfit and tearing into your foes with your Nightingale weaponry and Chillrend. Drop two perks into Smithing to improve your enchanted gear and you'll be the last thing they never see.
I see that my Skyrim builds tend to be my most popular posts. If you're new here, welcome. Come for the video games and stay for the crimethink.