Just bear in mind that all mapping app vendors are reselling Ordnance Survey (OS) 1:50k and 1:25k digital mapping data to you. You are either purchasing a fixed-term subscription (like for the OS app) to access the whole of the UK: when the subscription expires, you have no access anymore. Otherwise you are buying a set of map tiles for a specific area which are yours to keep in perpetuity (like the View Ranger app), but can’t be used on another vendors app. You have to decide which charging model is right for you, so use the free trials to help decide which app you find is easiest the to use and has the functionality you want.
As far as I know, while you can view OS mapping free in a web browser on many sites (e.g. Bing, WalkHighlands, WalkLakes, etc) you cannot download and save map tiles to your phone to use while navigate on the hill, apart from saving a screen shot as a static image. So when you inevitably lose the phone signal, you lose access to the mapping site.
One other advantage of apps like Locus and View Ranger over the OS app is that you can get Harvey Maps too, and if you hike outside the UK you can buy and use other country national mapping e.g. French IGN without having to learn yet another navigation app.
I use the OS app as a back-up for my Garmin GPSr and the app IMHO is good value for money and easy to use. You can take advantage of a free one-week trial or subscribe for a month at a time or a whole year: check out the OS online shop.
One other point which hasn’t been mentioned so far is that when you purchase a shiny new OS map today, it comes printed with a one-time pass-code which lets you download a complimentary identical digital copy of the map to be used in conjunction with the OS app. This is located on the inside cover and covered with a scratch-off coating (check the coating is intact when you buy!). So you could download the OS app for free then buy a new OS map in the knowledge that you will be able to have the identical map on your phone at no extra charge.
If you are still using a paper map but do not have the mapping on your phone, install the free OS Locate app which will use your phone’s GPS location to give you an OSGB Grid Reference which you can use with the map (no phone signal required). The app has a compass as well.
OpenStreetMap
Also, it would be clearer if we all used OSM maps to refer to the open source mapping derived from the global OpenStreetMap project.
There are many maps which are compiled using mapping data from OSM, some are free and some are paid for. Free ones include: OpenCycleMap, OpenTopoMap, HikeBikeMap, OpenBusMap. For walkers, OpenTopoMap has added contours and walls, and has matured to the point that in my experience it is generally usable for navigation. Go to an area you know and check it out.
For areas you don’t know, compare it to a map and look at aerial imaging of the same area on Google Earth. The Cicerone blog "Want more walks - navigation in a digital world" will give you some ideas of how to do this.
Paid maps using OSM mapping have other data added for the specific map purpose. A good example is by Martin Overton who runs the Talky Toaster website. He does both free OSM maps and paid-for ones where he has added value with lots more detail for hiking. (These are formatted for use with Garmin GPSr devices, so are not helpful for Android users, but the site will give you a good understanding of the differences in terms of what you might be getting by paying for a map derived from OSM data, and how close they are now getting to OS maps.