It's not the type of leather used in boots and the Brooks saddle that is different, leather is leather. It is the process used by Brooks in order to form the saddle that is important. This process used to shape the leather around the metal former means that the all the natural oils are leached out of the leather. It becomes wood-like, as is very noticeable in a new saddle. The leather fibres are now dry which means that the rider's "sit-bones" can now break these fibres in order to create the indentations required while leaving the rest of the saddle hard. This situation is necessary if the Brooks recommended method of breaking- in the saddle is to work. If oils remained in the leather, then the fibres would slide over each other rather than breaking and no indentations would be created, the saddle would then operate as a hammock.
The downside of this method of production is that, over the years, the leather dries out and cracks, albeit it may take 30-40 years. This can be avoided by a fairly light application of leather conditioner (e.g.Hydrophane .) on the underside, which will ensure that the drying-out process will not happen. Too much conditioner will soften the leather so care should be taken.
Personally, I have developed my own method over the years for my saddles and many others. Before fitting, I soak with water the underside of the new saddle in the area of my "sit-bones". The leather now becomes very pliable. I then use my thumb to create the depressions needed by my "sit-bones", (you can use foam or corrugated card board to find their position), and then leave the saddle to dry for 24 hours. The saddle will now be as hard as before but has the depressions. I now apply one coating only of leather conditioner to the underside, treat with Proofide on the top-side and polish off, and fit the saddle. The saddle is now ready to ride but you have no uncomfortable "breaking-in" to do.