This isn't something made clear by Odeon on their website, so I'm going to explain it both simply and extensively.
IMAX is a branded and marketed product with absolutely massive screens, iSense is Odeon's own brand and creation to create something similar with industry standard equipment that doesn't have "IMAX" written on it. For most people, there won't be much in it, though the selection of films available for each will vary (i.e. some films made for IMAX may not be in iSense, and the other way around. iSense screens are large, but not IMAX large.
When you see a film trailer, you will never hear "see it in iSense", but you may hear "see it in IMAX". That's because IMAX is an international brand that has a finger in every filmy pie from cameras to cinemas. I won't go into the film production side, but that's quite interesting. Instead, I'll focus on the cinema side.
IMAX, as a brand, has strict rules on how screens marketed as "IMAX" screens operate and behave in order to maintain consistency between cinemas (including different cinema chains, as it's not just Odeon with IMAX screens, e.g. the Cineworld in Ashton-under-Lyme and the Vue in Cheshire Oaks).
IMAX Digital screens have two IMAX-branded 2K projectors projecting onto the screen at once, IMAX Sonics sound systems, IMAX screen...you get the idea. Everything is branded and comes from one place. "proper" original IMAX is film-based and is usually one film projector rather than two digital projectors. There's not many of these in the country any more, and most films don't make film prints any more, so digital projectors are also used in these cinemas, and a rail system is used to swap the projectors around.
iSense, however, is Odeon's way of having a similar experience to IMAX, but without the costs of getting all the branded equipment and maintaining it to the level IMAX wants, which makes it more cost-efficient for smaller cinemas. iSense uses industry standard equipment, such as a single 4K projector and Dolby audio. It's still a "large screen experience" (i.e. a floor to ceiling screen thing), just with different equipment. However, this isn't the end of the story.
IMAX screens can only show films that are "made for IMAX". In other words, you can't show any random film made in super high resolution. iSense, on the other hand, doesn't have those restrictions. If a film is made in 4K, it can show it, as the only people they have to answer to is Odeon themselves. This means that the films shown in iSense can be much more varied than those in IMAX. Pretty sure somewhere was showing Bridget Jones's Baby in iSense, while IMAX wouldn't touch that with a 10 foot barge pole (not enough action and explosions I reckon). A quick check on Odeon's website, and they're currently listing the following films as showing in iSense and not IMAX: Moana, Passengers, Sing and Arrival. Moana I would definitely like to see at 4K, as that film already looked great on a standard film screen.
Here's something interesting. IMAX Digital, with two 2K projectors, creates an image at a percieved 2.9K, not 4K. This means that, in theory, iSense will have a better picture quality at a constant 4K. Of course, original IMAX with 70mm screen is a theoretical 12K (I say theoretical as it's film, not digital, so not measured in pixels). It does depend how the film was produced though, as the quality of the cameras or output will determine image quality, so your milage may vary.
I hope that clears things up. You'd be surprised how difficult it is to find a straightforward explanation on what iSense is, as I guess Odeon don't want to talk down their IMAX offerings. That being said, until recently there was only two iSense screens in the UK. Now, Odeon are building new iSense screens including one in Northwich, which opens next week. I suspect they will start making more noise about iSense in the coming few years as the ratio of cinemas with IMAX vs iSense starts to even out.