Partly it depends on what you've got right now, especially with soldiery (since it's not capped). For poetics, I'd go with whatever piece was the best for your class in absolute terms. Then go for the biggest upgrade compared to your current gear with soldiery (e.g., replace an i49 ring before replacing i70 boots, even if i100 boots offer more overall stats than an i100 ring). The higher you get your average iLvl, the easier it gets to farm soldiery.

If your play time isn't going to let you farm soldiery at a decent clip (e.g., you just have time to cap poetry each week and no additional soldiery past that), you might consider doing that with poetry, too, just to get your iLvl as high as you can.