The batteries you get from HD are real good batteries so I would stick with it. You may pay a little less for a aftermarket battery but it may cost you in the long run. I replaced my battery on my 08 about 3 months ago, the original battery was still good but I just felt better having a new battery in the bike, but I am a preventive maintenance type of person. On a side note I had the nylon strap that is under the battery that assist in the battery removal break, the strap was dry rotted so I had to make a new strap, I used a heavier wider strap this time. The battery is a real PITA to get out without the strap.
08 FLTR candy red sunglo, 103 Stage II, Woods TW-222 cams, V&H Dresser Duals with Monster Rounds, Thundermax tuner, Willy Shiny Wheels and Forks and a bunch of other stuff.