I would not use any other alarms than Delkims, although I have ST's with Atts dongles and receiver, and having worked in a tackle shop with the choice prefer not to go to Fox alarms.
You can change the sensitivity, no problems, I nearly always go to +on and then to maximum sensitivity. If false bleeps occur I wind it down, and to be honest, I can't ever recall using -on.
Bear in mind my Delks get used for carp, bream, tench, roach and pike fishing.
A couple of Delkim advantages, batteries, PP3 squares (try to ALWAYS use Duracell) are available from most places, Fox batteries can be alarm specific, tackle shop or specialist shop with some of their sizes.
Delkims are the same since the original launch. The alarms look the same, current ST's are still the same looking as 1994 ST's, TXi's the same as their version, and EV's the later releases, even if the components have been updated. With Fox, your alarm may not be the same model in two years time.
Indicator wise I have Solar indicators, Titaniums, before them I had Quiverlocs. They work as swingers or tension arm, and I can use them with semi-fixed, running leads and slack or tight lines.