i've done both parties and can assure you hall parties work out far more expensive than soft play! I would recommend soft play as its more for the kids - thats the point i suppose?.
If you do hire a hall you could have ball pit, small bouncy castle, slide with mat, some sort of bucket with water, cups etc to play with although messy!! In soft play you tend to feed only the kids hence it will work out loads cheaper than feeding adults. I have been to one hall party where they ordered pizzas and got the local chippy in - but depends if you're going out to impress!