I've lived in both places. I don't think you can go wrong either way. Here are a few observations about the differences, based on my experience:
Housing - As a single person, Tacoma would work out better for you, in terms of cost of living. Living in PDX has gotten very expensive for housing, which is usually a main expense.
Taxes - Also, don't forget about income tax, it really makes a difference. Oregon has about a 9% income tax, while WA has none. OR has no sales tax, and WA has a 9% sales tax, but if you are in your earning years and a net saver, WA is going to end up being a way better deal.
Lifestyle: Portland is still a magnet for younger people looking for a certain hipster (for lack of a better word) / "soulful" / artsy lifestyle. There is some truth to some of the comedy in "Portlandia". Actually, being from the upper midwest, you might interpret most of the coastal northwest (from Bellingham to Eugene) in that light. But it's probably most intense in Portland. If that's what you want to experience, go for it. You only live once, and as you say, you're young and single.
Be aware... In Tacoma, as a young, single, urban professional, you might be a bit of a fish out of water, and you might find the dating scene challenging, for example. I don't see your demographic as being very visible or present in the Tacoma area, except maybe at JBLM (a large military base south of Tacoma). Most of the large employers here in T-town skew to older employees with families (gov't, edu, health) You're actually more of a Seattle demographic. But I wouldn't plan on living in Tacoma and building a social life in Seattle. It may not look far on the map, but believe me the traffic is crushing, and not worth doing a "social" commute. (Traffic is really bad in PDX as well, but if you live centrally, you can avoid it for the most part. Same with Tacoma.)
Career - One overriding question might be whether there's a career / pay / upward mobility advantage of one location over the other for you. For example, if PDX is a central hub for your employer, and Tacoma more of an outpost, you might just go with PDX for that reason. (If your company is tech, PDX might be the hotter place; if it's import / export, maybe Tacoma is the hotter spot.)
Weather - Weatherwise, Tacoma is a lot nicer, imo. Not as hot and humid as PDX in the summer, not as cold in the winter. But being from the midwest, it's all going to seem pretty mild and easy to you, both summer and winter.
Outdoors - I would say both PDX and Tacoma are equally close to all of the outdoor stuff that everyone in the NW seems to claim they like, but almost never pursue :) I personally would give the nod to PDX, as it's closer to some spectacular open ocean coastline. But really, Tacoma is just as good.
Travel - If you travel a lot (like monthly), dealing with the airport as a Portland resident (light rail from most anywhere to the airport) is much easier than dealing with the airport as a Tacoma resident.
Good luck where ever you land.