iPhone 4 or 4S?

I'm blind and cannot afford an iPhone 5. I need to choose between the 4 and 4S. Specific concern is turn by turn for driving with maps and, whether Ariadne for walking directions will work on 4. How many maps can I store on 4? I live in a small town and only need location identifier plus just 2 or maybe 3 area maps at a time. I don't want to pay for expensive map or nav apps unless there is one I really need. I want to run the phone on StraightTalk or maybe Net 10 if that matters. I probably only need a dozen or so apps, and basic internet; I would only store a few small text files and maybe 2 or 3 added ringtones on the phone, no music files no videos nothing like that. I do want to use Twitter. I can live with slower speed but do need navigation capability; for both walking and driving, turn by turn. Can 4 be enough or do I need to go ahead and get 4S? I am unclear about whether the cost markup is really worth it for my needs. This will be my first smartphone. I only have experience with standard older model cell phones. Am using TTS on my computer for this forum. Suggestions very much appreciated!