Tickets in the stalls ("orchestra" in US parlance) are closer, but tickets in the dress circle ("mezzanine" in US parlance) are higher up, so you often get a better view of the whole stage at once. Particularly for spectacles like big musicals, it can be easier to take it all in without swiveling your head constantly. If you sit too close in the stalls, you'll think you're at a tennis match, but of course, further back in the stalls is fine too.
As for what to see, I agree with everyone else that it's too personal a decision to leave to others. Read about the shows, try to get the cast albums out of the library or listen to excerpts on YouTube or Spotify. Which one interests you more?
As for your hearing problem, shows now have various devices available. For instance, on the Wicked London official website https://www.wickedthemusical.co.uk/london/your-visit/access, under "Access," it says the following:
Deaf or Hard Of Hearing
The Apollo Victoria auditorium is equipped with Sennheiser MobileConnect WiFi sound amplification system. MobileConnect uses WiFi to deliver superior quality audio to a smart device such as an iPod or iPhone, either through headphones or via a necklace for hearing aid users. The sound levels can be adjusted on your device to suit your needs and our access host will be happy to assist you with this. Patrons are advised to book Stalls A-P for the best signal.
Devices can be booked via the box office in advance, borrowed on the day, or alternatively patrons can download the Sennheiser MobileConnect app using the links below to use on their own device.
On the Les Miserables London official website https://www.lesmis.com/london/tickets, it merely says the show is fully accessible, without giving details. It does say you can email [email protected] with questions. I assume they have similar provisions to Wicked, but it's worth checking.
Note that these official websites are the best place to get full price tickets. Other websites can add unnecessary markups, even though they sound official (such as, in New York, Broadway dot com, which adds a huge markup to the one already added by Telecharge or Ticketmaster). As a bonus, on the websites you can often see the view of the stage from the exact seat you are considering buying.