anything for $6k will be on its last leg

At that price point right now, your options are gonna be pretty rough. I can’t really comment on the Nissan, but I agree with doc about staying away from old colorados. 

If I had to buy a V6 4-by pickup for 6 grand, I’d look for a second gen S-10 with the 4.3 and 4L60 transmission. Good drive train, parts are cheap, and if you can find a southern truck without rust it might hold for a few more years. 

Sorry to say, but used pickup valuations are bonkers right now. Whatever you get, it’s gonna need some TLC. 

I'd go with the Frontier or recommend an old generation Ford Ranger with a standard. There's a reason you see the Rangers everywhere compared to Chevy S-10s and older Colorados. They're pretty reliable for domestics and sold like crazy. If you ever need or want to replace stuff, they're in junkyards everywhere. The engines were used in tons of different Ford vehicles, so parts availability doesn't just depend on junk Ranger availability, and the manual transmissions were designed by Mazda. Their biggest drawback is rear frame rust. Check for rust if you look at one.

The automatics aren't as good as the manuals, still, mine went 251k over 19 years with the tranny having never been touched. I had the tranny rebuilt 25k ago and it's still going 300 miles a week. My only regret was finding Scotty after that rebuild job because I can tell it wasn't done right and I paid close to what a Ford remanned unit would have cost.

Never buy a Colorado.  You'll be sorry.

What year? How many miles?