Tempeh and seitan are routinely listed together as meat alternatives, which obscures how differently they are made. One is a whole food held together by a mould; the other is a single protein separated from everything around it.
Tempeh keeps the bean intact
Cooked soybeans are inoculated with a mould and held warm until threads of it grow through the mass, binding the beans into a firm cake.
Nothing is removed. The beans keep their fibre, their fat and their structure, which is why a slice shows individual beans rather than a uniform block.
The fermentation also changes flavour, developing savoury and slightly nutty notes and reducing some of the compounds that make plain cooked soybeans hard to digest.
Seitan removes everything except the protein
Wheat dough is washed under water until the starch and bran have gone, leaving only the gluten proteins behind as an elastic mass.
Those proteins are then stretched, folded and cooked in liquid, which aligns them into fibres that pull apart in strands rather than crumbling.
Because the starting material is close to flavourless, seitan tastes almost entirely of whatever it was simmered in, and it will taste of nothing if that step is skipped.
They behave differently in the pan
Tempeh is porous and dry, so it absorbs marinade slowly at the surface and browns readily because there is little water to drive off.
It also holds its shape under long cooking, since the mould network and the intact beans do not soften into a paste the way a purée-based product would.
Seitan is dense and wet and resists absorbing anything after it is cooked, which is why flavour has to be built into the dough or the poaching liquid rather than added later.
The texture they imitate is not the same texture
Seitan reproduces the pull and chew of muscle fibre convincingly, which suits preparations where tearing and stretching are part of the experience.
Tempeh does not attempt that. Its texture is closer to a firm, granular cake, and it works where a substantial bite and a savoury edge matter more than fibrousness.
Trying to make either do the other's job is the usual reason a dish built around them disappoints.
Their constraints are opposite
Seitan is made of gluten, which rules it out for anyone avoiding wheat protein, and it contributes very little beyond protein and chew.
Tempeh is a whole legume product with fibre and fat intact, but its assertive fermented flavour asserts itself in delicate dishes rather than disappearing into them.
Choosing between them is therefore a question about the dish rather than a question about which is the better substitute.