In general, I would not recommend spirals for letters. Text is one of the most challenging things to scroll well, especially if the font is mostly straight lines and sharp corners. Any divergence from the pattern line will be noticeable, because everyone knows what an
E is supposed to look like. Tough to do with a spiral. If the font is more abstract and the material being cut is relatively thin, then perhaps a spiral would be appropriate.
What thickness and what type of wood are you cutting? The combination of the pattern, wood thickness and hardness all work to determine the best blade for a particular application. Not knowing all of these details makes it hard to give a specific recommendation. Personally, on most woods up to 1/2" thick, I like to use a #1 or #3 Flying Dutchman Ultra Reverse blade. Generally speaking, the thinner the stock, the smaller the blade. Straight blades with reverse teeth will provide the cleanest cut. Beyond that the specific brand tends to be a matter of personal preference.
This is an example of a portrait (not mine, but I've done similar ones). Granted a pretty advanced one, but typical of portrait style patterns. These patterns lend themselves well to using spiral blades because if you look closely, there are very few straight, smooth lines. Small deviations from the pattern line will be mostly unnoticed. Spirals work well when the project is large and would be difficult, if not impossible to spin and make all the cuts with a straight blade.
If you are going down a river at 2 mph and your canoe loses a wheel, how much pancake mix would you need to shingle your roof?