Poaching a whole fish: kettle or foil?

There's not really a whole lot of difference here; in either case you're talking about poaching a whole fish, it's just a question of equipment.

The fish kettle is a quite specialized piece of equipment, so the main drawback I can see is: where are you going to find one? I can't think of any restaurants I've worked in that had one of these (and good luck getting them to loan it to you if they did). If you know where to get one that will fit your fish, then the second drawback you've noted is that you'll need to clean it after use. There are two main advantages with the fish kettle: it's spacious enough to accommodate extra cooking liquid if you choose to add some, and since it's a solid piece there is minimal risk of liquid leaking out.

By contrast, aluminum foil is readily available, though you need enough of it and a large enough size for a double layer underneath the fish with enough to wrap and seal over the top. Since the metal is thin enough to tear easily, and you're "building" it to suit the fish, there is always the possibility of leaks or tears. For this reason I wouldn't recommend adding more poaching liquid like you can with the fish kettle. Placing your aluminum foil package onto a sheet pan if available would help with transport and any small leaks that may occur. Since the foil is thin, your package might heat through marginally more quickly than a heavier solid kettle, but probably not enough to affect the cooking time.

If you had a smaller fish, you could also consider a large, solid baking pan with high sides:

This has some of the advantages of both kettle and foil: they're readily available, solid enough to minimize the possibility of leaks, and deep enough to add liquid as desired. You'd need some foil to tent over the top unless you can find a dish that comes with a lid, and unfortunately they don't tend to come much larger than about 13 by 9 inches; this will almost certainly not fit a 5-kg whole salmon. I mention this mostly as a generic answer for smaller fish, or in case you're considering cutting off the head/tail or cutting fillets instead.

A side note regarding the dishwasher: while this is possible as demonstrated in the linked thread, the demonstrated technique is really closer to sous vide cooking than traditional poaching. If you're willing to consider the dishwasher you could also consider a full sous vide approach, assuming of course that you have a sous vide heater and a large enough vessel/bag.

A whole salmon of 5 kg is going to be about 75 cm long. Very few, if any fish kettles are going to able to accommodate a fish of that size whole. If you find one big enough, it is likely to cost several hundred dollars (US), hundreds of GBP, or hundreds of Euros. Are you going to want to spend that large an amount of money in relation to how often you with use it?

The next problem you need to answer is, will a fish kettle of that size fit in your oven? I just measured mine, my oven won't accept anything much longer than 58 cm maybe

I would opt to cook it wrapped in aluminum foil.

Use foil in the oven, it's easier, and even if you over cook the fish a little bit it will still be moist.

Put the fish on a double sheet of foil on a baking sheet (for easy transport).

Put herbs, spices, lemon (or whatever you like), in and around the fish.

Wrap wrap the foil so that air will not leak.

Do not wrap the fish tightly or the steam might tear up the aluminum foil.

Cook in a high temperature oven, at least 400F, maybe 20, 30 minutes, a 5 kg fish is big.