I am an Architect (need one?
--just joking!!) and my advice is it depends on what you want. If you are really only after a stock house then go with the plan services. It will be cheaper and they provide a good set of floor plans that most builders will be comfortable with.
If you truly want a custom designed home to your needs/wants then hire an Architect. An Architect is your ally in the construction world and can save you a lot of money but he/she comes at a cost. You have to weigh whether that cost is worth it to you. I think it really depends on your personal background. If you have little or no experience with a contractor it will be difficult to discern if what he is telling you is true. An Architect is in your employ and is your advocate and voice during conflicts and interpretations of issues. I would also be your advocate with the zoning/city process--often a daunting task for most people.
I have done services across the board for clients from simple plan sketches only, to complete services including construction administration. It all depends on what you need and want and are willing to pay for. I believe I provide a valuable service for my clients but it is frustrating sometimes as many people will happily pay the real estate agent who sells their house 6% but begrudge the designer anything in that range.
My advice--Whatever budget you have, double it!
Well not exactly but it likely isn't enough as most people have not taken into account the myriad minutia involved in a building. And are shocked at the cost of say 20 joist hangers, or 20 hurricane straps, etc that they hadn't thought to include in a preliminary budget for example. Nor are most people prepared for the decisions that are required during the design and construction process.
Also you and your partner need to be in a mental state where you can make decisions and stick with them. Time is money to both the Architect and the Builder. If you can't decide what color to paint the trim and want to see trim painted with several choices before you can make a decision, that will cost you. Your lack of a quick decision will directly translate to dollars as the builder and his sub are not going to paint that trim for free and stick around while you decide if you like it or not. You need to take the professional advice of your Architect, then quickly decide and then give direction to the Builder. If you can do that you will save money, if you can't you will cause huge stress (I mean HUGE STRESS) in your relationship with your SO and it will cost you lots of $$$$.
In my humble opinion most people can't understand three dimensionally what the floor plan means and that is where I think an Architect would be helpful for you as we have 3D tools such as SketchUp and Revit to model the building. It is much cheaper to have me build it in cyberspace then in concrete and wood!
I am not going to tell you that you can't do this without an Architect as it is done everyday. Some more successful then others granted, but hiring an Architect is no guarantee of success in the perfect building that you wanted.
Feel free to ask me anything either her or via Private Message. My experience is more in the high end multi family area, but I have done my share of single family houses as well.
Either way---good luck! Actually my wife and I are in the process of deciding if we want to design and build a new house on a piece of property we own...just this Architect I am working with.......