ID90 v Hotline pros and cons:
ID90 fully flexible and cancellable.
Hotline possible to change after midnight UK time on the day of travel. Such change can be free but may also incur a fee.
Hotline cancellation only refunds the APD.
Sometimes Hotlines are cheaper than ID90s. Even if the particular flight that you want is more expensive, you can book the cheapest flight and then change it on the day to a better time, normally for free.
Hotline tickets give you full commercial passenger rights. Your seat is confirmed and you can normally choose a seat for free in advance. You can check in 24 hours before departure. You also gain avios points and Club tier points (not many on LHR-CDG!). You can clear through departures and security as soon as you like.
ID90s are issued on a “standby” basis; i.e. if space is available. No remote check-in. You must physically check in at LHR by STD-45 at the latest and then it’s a lottery whether or not you’ll be allocated a seat or whether you’ll remain on standby. Sometimes you have to plead with staff to get an “airside indicator” so that you can get to the departure gate. LHR departures security-gate cut-off is STD-35 which can be a bit stressful. Contact with the operating crew is useful as they can authorise a jump-seat which gets you over these hurdles.
In summary, I try to use Hotlines as often as possible as the experience is altogether more pleasant. The only time I would use ID90s is if I’m unsure of my plans and chances of changes/cancellations are high. Also, sometimes there just aren’t any cheap deals on Hotline, normally because I’ve left it too late.