OEM vs Aftermarket replacement parts: Which one to buy

What is an OEM part?
An OEM part is the exact part by the same manufacturer whose part is originally fit inside a brand new vehicle or machine.
The quality, design and all dimensions of an OEM part will be exactly the same as the one that came with your brand new vehicle.
For example, an OEM starter generator for a golf cart is the same starter generator that was fit in that cart when it was bought.

What is an aftermarket part?
An aftermarket part is a part that is not sourced from the original OEM maker but made by another manufacturer.
Although it is not the same part that comes with the brand new vehicle, it is designed to fit your vehicle and perform a similar or even a better job than the original.

What is the difference between them?
One of the main differences is that you can generally be assured of the quality of an OEM part, but you need to choose carefully between the variety of aftermarket options available.
Many aftermarket parts perform same as an OEM, but there are also those which do not perform up to the mark.

Here is a short list of pros and cons to help you make a more informed decision when choosing with which part to replace in your vehicle. 

OEM Parts

PROS 
Assured Quality: No need to worry about the quality of an OEM part. Quality is guaranteed almost always.
Longer Warranty & longer life: Most automakers back up their OEM parts with more than one-year warranty. They usually work for many years before wearing out.
CONS
Quite Expensive: OEM parts will usually cost a lot more than an aftermarket part.
Poor Availability: OEMs are usually bought from their retail dealers & are hard to find online, so there won’t be any customer reviews to compare & they can’t be delivered to your doorstep.
Quality may not be superior: You buy an OEM part from your hard-earned money, hoping that it would be vastly better than an aftermarket part. But that may not always be the case! Some aftermarket parts are equal to or in some cases better than OEM parts. So you might be paying extra just for the brand name.

Aftermarket Parts

PROS 
Less expensive: Aftermarket parts come at almost half the price of an oem part!
Quality can be same as oem: Many aftermarket parts perform similar to an oem. Good aftermarket manufacturers reverse-engineer the part, and work the weaknesses out.
More options: While there will be only one OEM manufacturer, you can find dozens of aftermarket alternatives to choose from.
Better availability: Many aftermarket parts are easily available online and can be ordered at the comfort of your home. Trustworthy aftermarket companies provide a strong Warranty cover & good customer service for a hassle-free experience.
CONS
Quality varies greatly: The saying "you get what you pay for" may ring true here. Some aftermarket parts are inferior because of the use of lower-quality materials.
Overwhelming selection: With so many options to choose from, one should know how to spot the best deal! Choose the part that has good reviews online, provides a one year warranty at least & picks up your call on calling their customer service!
May not have a warranty: To keep costs down, some aftermarket parts are sold without a warranty or the seller may not honor their warranty promise! Hence it is important to buy from a seller you can trust. Call them to see if they are responsive.

"A trustworthy seller to buy starter motors, alternators & starter generators for pickup trucks, tractors and golf carts is www.partsworldusa.com. They provide high-quality aftermarket parts at best prices in the market & have a great customer first ethic."

 
VERDICT:
To sum it up, OEM products have better quality in general, but aftermarket products are a lot cheaper and they can get the same job done, if you buy from the right seller!
Now that you understand the difference between OEM and aftermarket products, you can be a smart shopper of replacement parts.