Amp under the seat - usually much easier to bolt down. Head would need a fair bit of faffing to get it solidly fixed and connected.

Also amps are usually designed to be hidden away - head units usually need buttons to be pressed etc.
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Rory
MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
MkII, 240Gb in Mark Lord dock
MkII, 80Gb SSD in dock