I have my first Spaniel who came at 2 completely untrained, I've also got a JRT.
The JRT is a gobshite if allowed to be and will alert you to nothing anything, she's not got a yap but a bark and sounds bigger than she is. I wouldn't advise for a first time owner though, or with small children, they're little size wise but they've got big attitudes and without stimulation, training and supervision can be awful (I've met a few) and snappy. And it may just be mine but she'd not think twice about sacrificing me to save her own furry arse.
I call the spaniel the muscle behind the JRT's gob! She watches and weighs up a lot and she positions herself between me and anything she doesn't like the look/sound of, until she's told it's ok, and watches, alert.
We were approached and confronted by another dog and she kept manoeuvring to keep herself between me and that dog that was circling and was hackles up, growling and barking. I've never had a situation like that with a human but I don't doubt that she would react the same way if we were confronted by a human who showed bad intent.
She barks at someone at the door/coming into the garden, but comes back to me for reassurance, unless she knows them and then it's all belly rubs and wagging. If she doesn't know them she stays close to me but isn't aggressive, just watchful until she knows them a little and realises I'm relaxed.
Wouldn't say a working spaniel would be for a novice owner, but most dogs if you put in the effort with the relationship will protect their home and family as a natural instinct - but you've got to train and work at them being a sensible and appropriately behaved member of the family or it could quickly turn sour.