English is definitely the easier to start a horse out in, then move to western. Keep in mind, that it does not matter if you have a western saddle or an english saddle on the horse. We start all our horses out in a western saddle, regardless of if they are saddleseat, hunter, or western- there is just more security in the western saddle.
By starting the horse english you will not be tempted to started pulling to get the horse to slow down. You work the horse back to front (as with all disciplines), with a young horse it is easier to allow them to move out a bit, esp for balance. Once the horse understands to work from behind, come onto the bit, and all your cues, you can begin asking them to collect and slow down a bit. Really, you can incorporate some jogging and a few strides of loping when working your english horse. This will introduce the concept of jogging and loping, but will not put such a demand on them physically or mentally. It will also increase their responsiveness to your seat and other cues, thus making it much easier to develop them into a western horse as they mature physically and mentally.