Adding a new fence to your property can give new life to an old yard, and the type of top rail that you have is something people are going to notice easily. But the type of top rail you have can also affect issues like security and the stability of the fence. You'll see two common types: straight or spiked. Choosing between them is straightforward.
If you have a lot of wild animals in your area, particularly deer, avoid spiked fences. Go with a flat railing (thick or thin) that won't impale the deer should the animal misjudge the height of the fence. Unfortunately, it has become common for deer to be injured as they try to leap over spiked iron fences. Try to go for a thicker or wider railing because if the animal doesn't jump high enough and hits the top rail, a wider surface might cause less injury to the animal.
However, if you have pets like dogs that keep trying to jump out of the yard, a stable, solid top rail might not be best. The dog could use it as a step. In this case, you would want a spiky top rail, but with the spikes encased in brackets that cover the points of the spikes and that don't form a solid line that the dog could treat as a ledge.
Pressure on Fence
Lower fences can serve as places for people to gather and gossip, and a wide, thick top railing on a shorter fence could allow people to rest their arms as they chat (thin straight rails work too, though they'd be less comfortable). However, the added weight from everyone leaning against the fence can make the fence itself lean or weaken. If the fence you're getting is going to be installed in such a way that it can handle people leaning against it, a straight top railing is perfect. If you prefer that people don't constantly lean on the fence, spiked railings should work.
There's no doubt that the harder you make it for someone to get over the fence, the harder it will be to reach your house if that person is trying to break in. A spiked railing on a tall fence is best for this situation. If you want a straight railing, though, make the fence solid so that there are no decorative flourishes or bottom rails that could serve as footholds for someone trying to climb over.
You're going to find many different designs for fence rails, so take your time and try to see samples in person. If you need those covering brackets for spiked fences, look specifically for animal-friendly (sometimes called deer-friendly) fences. Talk to companies such as Bracci Fence Inc for more information.