I don't have a 15 series, but a 25 series, same as Eric. Only difference between mine and his is that mine's cabbed. I'm still very happy with my cabbed 25 series.
The most important thing for you to factor in is what tasks do you want/need your tractor to do for you? There is no wrong answer here, and different folks will have different answers. Your tractor is a tool. It can move/carry/lift "stuff" around to be sure. But it can also "power" stuff. Attachments like PTO driven mowers, tillers, chippers, snow blowers, pumps, generators... Lots of stuff. Just make sure you have your own needs in mind when you select your tractor, as you may need more "power" than say another tractor customer. You may need less too, nothing wrong with that either.
That 3015 has 30 hp at the engine, and 28 hp at the PTO. Now the real question you have to answer is, does that 28 hp at the PTO satisfy every need you will have for your tractor? Will it power every attachment you see yourself needing?
Assuming you're getting a FEL, does that have enough rated lift for whatever you need to pick up? Bags of feed/seed/cement? Fine. What about say, large round hay bales? Not only does the FEL rating have to match (or exceed) your needs, but what about tractor weight? Usually weight is your friend, and heavier is better. But not always. If you're having to travel over landscaped yards, heavy may be an issue.
You can always go up in hp, and stay in the same series. A 35 or a 40 hp unit will cost a few extra bucks, but open up more freedom for PTO driven tools (and their SIZE) you can effectively run.
Again, no wrong answer here. Just make sure the tool (tractor) you buy meets all of your needs, and you'll be happy.