Taking a side trip from Temecula for an overnight stay. 2 adults - leaving kids with grandma. What are your thoughts on one vs. other? Any lodging recommendations? B&B style/inn - maybe $100 (or slightly more is OK) for one night midweek in July. Or Idylwild?

Well, $100 a night is not going to get you much IMO. Certainly not in the B&B or Inn category as you described in the Big Bear or Lake Arrowhead area. But who knows? I could be wrong since I generally go during the peak winter season.

With that said, if you want to go higher (say $175-$200 a night), I recommend Lake Arrowhead and the Lake Arrowhead Resort. Perfectly located with easy walking distance to shops, restaurants and the lake. I have now been there twice and enjoyed my stays thoroughly.

Plus, since you are going to be limited on time, you might appreciate the fact that it is a lot quicker to get to than Big Bear.

I respectfully disagree with SSM. Big Bear is not "alot" further. It is a mere 30 minutes. Stay on 38 and don't take the arrowhead turnoff and there you are.

Big Bear, imho, has a whole lot more to offer than Arrowhead village which is basically a small outlet mall. $100 isn't out of line for a night in a cozy cabin mid-week in Big Bear Lake. Try the Grey Squirrel.

Activities:
Tons of fun shops in the village,

go to a non-g rated movie (in honor of no kids)

rent a boat or take a paddle boat tour of the lake.

Drive around the lake to the Big Bear Discovery Center.

Take the nice 1 1/2 mile Woodland Trail hike directly across from the marina on the north shore.

Visit the cute shops along Moonridge drive.

Eat at:
Grizzly Manor for huge breakfasts (on Big Bear Blvd.)Pancakes are the size of hubcaps, or try the Sissy-Boy breakfast.

El Jacalito on Pine Knot Ave in town for Fabuloso Mexican food and great margaritas

Blondies on Pine Knot Ave. in town. The best fish and chips ever.

We have a weekend cabin in Big Bear (in case you couldnt tell) and are up there year round. Love the mountain atmosphere.

Try bigbear.com for ideas and info.
Watch out though. Some places require a 2-night minimum.

No comparison... BIG BEAR!! We stayed at the Northwoods Resort! (Romance package)

We've vacationed in La Arrowhead every summer for the last 23 years- but I also like Big Bear. Both are nice. Big Bear does have more to do for tourists. It is not as pricey as lake arrowhead either.
If I was in your situation I would pick Big Bear personally.

We've been many times to both towns, but for one quick night in July, I'd choose the Resort at Arrowhead. It has a nice swimming pool area, with views of the lake. You can also take a tour boat ride around the lake for an hour. Can walk to village for dinner. In a lovely setting.