Once you render out your rasterized graphics to a fixed size you run into problems when using them as overlays. The difference in the behavior of rasterized anti-aliasing is huge as opposed to the near infinite resolution you get from "inline" type. If you export your graphics with a transparent background, there's really no telling what it will look like against differing backgrounds. Furthermore, if the rendered/rasterized graphics are scaled in any way, especially larger, it will become a mess.
Motion 5 (output) is designed to be integrated into the video, that is, you're compositing *into* the video/background. This works *if* you create your text as a Title effect and "publish" the effect to FCPX. In FCPX you apply the Title effect to the storyline (it's exactly like layering in Photoshop). Set the text IN FCPX (usually, although you can set it in Motion and it will be ready to go for you in FCPX.) Title effects in FCPX are capable of very fine resolution and subtle effect, for instance, embossed text effects. Furthermore, if you prepare your text as Titles for FCPX, it is a simple matter to change faces, fonts, and everything that goes with them right from the inspector, so that you can experiment with what works the best, visually and thematically. (And don't forget about Blend modes - everything you're used to in PS is available to you in Motion/FCPX!)
That said, Helvetica would not* be my first choice for a gothic face for video. I abhor Arial in print, but it works well in video, particular the black face; but Impact is much better. Forgotten Futurist and Franklin Gothic are also excellent choices (Stone too, but it's a bit more "informal"). I am personally partial to Myriad because the italics are superb (and "different"), as are the condensed versions. With HD video, we are no longer limited to the sans-serif class typefaces and the options are virtually limitless. Serif faces are considerably more readable than sans faces... but... we are slaves to our clients 😉
*All of us "children of the 60's" were bludgened to death with Azidenz-Grotesk/Univers/Helvetica, the "corporate typeface of choice" — and with lower res video (i.e. interlaced SD TV) — necessity.