Getting another dog when one has just died often feels wrong, like in some way cheating the old dog's memory by suggesting that another dog will be enough to fill the hole he left behind.
Getting a new dog too soon might be awkward, especially if you're going from a well-behaved old friend to an unruly and difficult pup. The extra workload in training a pup can make you wonder why you bothered getting another dog when he'll never replace the old one.
On the other hand, if you've always had a dog, your life might feel empty without one. It depends on how you cope with the grieving process and how well you fill the time that you would have spend going for walks, playing in the garden, etc., and the little things you don't realise, like when you're watching TV and he comes and lies on your feet, or when you walk into the kitchen and he's always there asleep in his bed.
Every dog is unique, and one can never replace another, but in time you grow to know each dog in its own special way. The grieving process is a perfectly normal human thing, and part of the love/bonding thing that we have projected onto pets as well as family and friends.
I'll never forget the state my old man was in when our German Shepherd died in his sleep one morning, with no warning whatsoever. He couldn't understand why he was getting so upset over "just a dog." But to us, they aren't just dogs, they're people, they have feelings and emotions and special languages that only we can understand.

EDIT: BTW, does your current dog play with your RC18T? Cos my Shepherd is terrified of my Tamiyas

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