I have stayed at both hotels in the past. I gather the Park Plaza Arena Hotel has been completely refurbished last year and I gather it's getting rave reviews this year. It certainly has the best situation in beautiful surroundings. The Park Plaza Histria is more expensive and the rooms were fine but really it cannot match the situation of the Plaza Arena. If the Park Plaza Arena had not had its refurbishment then I would have recommended the Park Plaza Histria, however with the refurbishment of the Arena and the recent reviews I would certainly recommend the Arena. The Arena has a lovely setting within pinewoods which gives shade in the heat of the day.. A path down to a very pretty beach and a short walk takes you to a very good beach bar/cafe serving good food. There are places around to eat. Pula is not really walkable from either of the hotels but there is an excellent bus service into Pula that stops nearby - it's only around 2 miles. If you have a late night in Pula there are plenty of taxies from Giardiani (the main street) back to Verudela.
If I were in your situation I'd definitely go with Park Plaza Arena. I would also go B&B. because local food is good and it's good to be flexible. What we normally do is book B&B and ask if we can arrange dinner on nights when we're lazy and want to eat close to home. Normally there's no problem.