Our spaniels kill mice (they stamp on them with their front feet) and rabbits, but have never encountered a rat, so I don't know how well they would deal with one.
Terriers (I am a terrier girl to the core, so I am biased) are the best ratters you can get. They are phenomenally alert, incredibly fast, totally fearless, and when focussed on their prey are not distracted by anything else. We've had 10 terriers over the years (most at any one time was 6) and they are such cheerful, loving little characters.
We have spaniels now and they are lovely, they really are, but need a HUGE amount of exercise - physical and mental. Whilst all of our terriers thoroughly enjoyed a good walk, but would then flop and cuddle in when you came back, two hours of wild running about just whets a spaniel's appetite for MORE! They will happily zoom around the house like a Wall of Death for another half hour. They are also less openly affectionate than the terriers (though I admit my experience is limited - and horrifically greedy! This makes them easy to train with a food incentive, but means that you can't leave ANYTHING unattended. Terriers are digger, though, and will excavate the garden given the chance.
Spaniels also need daily grooming and if you can't cope with this, regular clipping (£40 approx, depending where you are). They are lovely dogs though. Actually - all dogs love company - why not get one of each
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