Citadines is in the city center; close to upscale shopping malls (Grand Indonesia, Plaza Indonesia), and the hip Jalan Sabang area.
Harris FX is a good base to explore malls of Senayan area (Pacific Place, Senayan City, Plaza Senayan) and to access other parts of South Jakarta.
I don't know for sure by what you're referring to as "hip and trendy clothing" - is it must be a designer creation? Or independent labels from local clothesmaker? If you want to seek the latter; I suggest to explore few distros in Jakarta. There are 2 main distro area you can explore:
1. Tebet (Jalan Tebet Utara Dalam) is a street that lined up with distros (the shape is house-converted shops) with few eateries in between. Very lively with teenage crowds. From Citadines/Harris FX is around 30mins drive and the nearest hotel will be Harris Hotel Tebet (10mins drive). P.S. currently, there is a roadside construction to upgrade the road in the area, expect a rather heavy traffic jam. Avoid office rush hours.
2. Barito. Barito is housing area right behind Blok M Plaza and Jalan Mahakam. The distro here is scattered in between the lanes, hiding among residential houses. You can find some bigger one at Jalan Lamandau. Approx 20mins drive from Senayan area.
For young designer creation and other quirky stores, you might want to check out Level One at Grand Indonesia West Mall (that's the area around the defunct Harvey Nichols).