Depends on how much of a rifle loony you are.
As the gunsmith who barreled my custom 6.5x55 noted, the chamber dimensions of the 6.5x55 are "all over the place." This is because it's a very old military round, and like a lot of old military rounds, the original throat was VERY long to accommodate heavy round-nosed bullets. These days those are far from the most popular choice for 6.5mm rounds. Instead most people want to shoot spitzers with very high ballistic coefficients, which may not shoot very accurately in a traditional, long-throated 6.5x55 chamber. Which is why mine was chambered with a custom, short-throated reamer.
The rim size is also a little larger than the "standard" 8x57/.30-06 diameter, which occasionally causes problems with some bolts. Not usually, but sometimes. It needs a "long" bolt action, and some are a lot longer than needed for a 6.5x55. Mine's built on a commercial FN Mauser action, which is perfect, and a Ruger Hawkeye would also be about right; their CRF bolts also usually work fine with the rim size, but if not can be easily modified. While the 6.5x55 will feed fine in a long Remington 700 action, it's far longer than needed, and the case-rim may not fit in the bolt face.
I'm assuming you're handloader, so you'll also need to contend with 6.5x55 handloading data that's all over the place, due to widely varying chamber dimensions, plus rifles of widely varying ages and action strengths. If you want to buy factory ammunition once in a while, you're mostly going to find rather wimpy American ammo, which may or may not shoot very well in your custom-barreled rifle. Some factory ammo--both American and European--may not even fit in a minimum-dimension custom chamber, and European ammo may even be too hot for such a chamber.
On the plus side, Lapua (and other European companies) make 6.5x55 brass of excellent quality, and the 6.5x55 has somewhat more powder room than the 6.5 Creedmoor, allowing either a little more velocity, or the same velocity at lower pressure. It's also a cartridge with a lot of history and hence "cool factor," and definitely works very well. In fact, it was pretty much the inspiration for all the medium-sized 6.5 cartridges that followed.
The 6.5 Creedmoor doesn't have the "all over the place" problems, because it's a modern round with very specific chamber and case dimensions. It will work fine in any short bolt action for a .308-size round, and since the Creedmoor's popularity is increasing, both brass and very good factory ammo are becoming widely available. While velocity potential is a little less than the 6.5x55, the difference is less than 100 fps. In other words, it's by far the more practical choice.
The rifle loony way would be to own both (along with rifles chambered for several other 6.5's) but that's not particularly practical.
