I have just joined the forum. I received my Cyclotricity Stealth 1000w this week and have had one ride out. I live at the foot of the North York Moors in North Yorkshire, England. I was disappointed that I only managed 10.5 miles on my first trip out and it was extremely hard work pedalling the last 2.5 miles home unassisted. I didn't use the 1000w option (and beginning to wonder if I made the wrong purchase, as this will drink the battery even more rapidly).
If you'd have asked on this forum before you'd bought the bike, we'd have told you that. There's much better bikes for the similar money that go further, faster and get better hill-climbing.
You might find that you get more range in 1000w mode because the motor will run a bit more efficiently, though it won't make a massive difference.
You could sell the motor on Ebay and put the money towards a nice geared one. At 48v, the regular 250w ones will most likely give you more than enough power for what you want and a lot more range. If you have the standard 9-pin connector, most motors are interchangeable.