Depends on how much you want to spend, and what you want to end up with.
I have a Remage that started life as a 700 SPS varmint. It shoots great, and I really like it. Over time I've modified the rifle slowly, and it still isn't completely what I want. I want to get it cerekoted next. The NSS remage barrel cost me $325, plus about $300 gunsmith fees to true the action face, lap lugs, and remove the factory barrel (they are usually screwed on very tight) and install the new one. Now going forward, I can change barrels myself if I spend about $150 to buy the tools. Keep those costs in mind when making your decision. I did it this way because when I started, it was my only option to have a nice rifle due to not having extra money. It spread the costs out, and in the end I have a very accurate rifle. But now that money is less of a problem for me, I do sometimes find myself lusting over all the new custom actions hitting the market.
If having the ability to swap barrels yourself is important to you, then go for the remage without fear. If not, and you have the money, I would buy a custom action and build from there. It will cost more, but when its all done you'll have a full custom rifle, versus a souped up remmy. And there isn't much I can do to make my rifle as nice as it would be if I had started with a premium custom action.
Depending on what action you choose, I'm guessing the rail, rings, and trigger might all work with the new action. So not all your investment into the current rifle would be lost.