Although Frank has a great answer above, I thought I'd add a couple of things. The question of the proximity of a text to the original depends on a number of factors, age being an important one, but certainly not the only one.
To think about this, it is necessary to think about the process of manuscript manufacture in the early years of the church. Manuscripts were copied meticulously by hand, but like all manual processes they suffered from copying errors. There are basically two types of errors, deliberate copying errors where the copyist chose to write something that was not in his exemplar, and accidental copying errors where the copyist accidentally wrote something different than the exemplar.
Age is actually more a proxy for generational number -- the original was first generation, the copy was the second generation, the copy of the copy the third and so forth. Consequently, since the age of the manuscript approximates the number of times it has been copied, it approximates the generational number, and with each generation there is the possibility of the introduction of errors.
So the value of a manuscript is both dependent on the generational number, and the quality of the copying at each generation.
The age and consequently generational number is determined via a science called paleography, where the style of handwriting and the materials used are used to estimate both its age and its geographical origin. The ingenuity of the people who do this is quite remarkable. I'd definitely recommend learning more about it just to be amazed at what they do.
The two different error modes tend to be determined in two different ways.
Accidental errors are usually categorized into typical and common errors. For example, copying text from a preceding line, or skipping a line, or substituting one word with a similar spelling for another, or substituting Jesus words from one gospel into another.
The quality of a manuscript can be pretty readily judged by looking at non consequential errors -- how many unimportant errors did the scribe make as a measure of how many consequential errors did they make.
In terms of deliberate errors, where the scribe "corrected" the text, we are in more trouble. However, this is determined by looking at many different manuscripts, comparing them based on various criteria, and identifying where they are significantly different from the "mainstream."
However, majoritarianism does not win here. For example the majority of Greek texts are classified as Byzantine. Most early English translations came from the critical version of this called Textus Receptus. However, it is not widely regarded as a good reflection or the original because of the two modes of error mentioned above.
If you want to know more about this you absolutely have to read the introduction to Nestle and Aland's Novum Testamentum, which is the best regarded critical Greek text, and is pretty universally used, and/or Bruce Metzger's "A Textual Commentary", or final recommendation Kurt Aland's commentary on his own work The Text of the New Testament
So the answer to the question is simply that Sinacticus is a pretty faithful rendition of the original, but Nestle and Aland is better. Nonehtheless, as I have said here before, everyone who can read Greek absolutely should get his head out of the clean, sanitized version in N&A and look at some of the early texts, such as the beautiful Sinacticus, or Chester Beaty papyruses. It will give yo ua whole new perspective.
BTW, I did not even address the question of what does "original Greek New Testament" mean. Questions like the originality of John 21, for example, are far from trivial.