build quality
reliability
adjustability
battery usage
longevity
backed up by a quality company that just specialises in bite alarms and isn't just re-badging imported foreign shit
Like anything in fishing, you have to justify the expense, ?100 odd pound each for Fox ones, or the same price for Delkims, in my mind is no contest.
?100 odd pound or ?25 for a cheap one, is down to how much you use them and for you to justify.