For definitive halachic advice, CYLOR as usual.
See B'rachos (29a) where Sh'mu'el HaKatan forgot the text of the prayer that he had just instituted (V'lamalshinim), and he stood there for two or three hours until he finally remembered the formulation. If you're not confident in your memorization, and you think it is possible that you might forget some of the text, it's probably better to wait for a siddur (a luxury to which Sh'mu'el HaKatan did not have access) so you don't end up in a quandary where you are in the middle of the amida but can't remember what to say.
As far as reciting a version other than as established, the general rule that "anyone who departs from the formulation established by the Sages has not fulfilled their obligation" (B'rachos 40b) also applies to the Sh'moneh Esrei (Sefer HaManhig, §66; Beit Yosef, OC 122).1 2
See this article by R' Doniel Neustadt, which discusses the halacha for if you are in the middle of the Sh'moneh Esrei and find you are not sure of the correct words, but you know that a siddur is nearby:
If one begins davening without a siddur and suddenly requires one in order to continue davening properly, he may go and get one if he knows its exact location. He many not, however, start searching around for a siddur (Rama O.C. 96:2, according to the explanation of Chayei Adam 22:7; 25:9 and Mishnah Berurah 104:2. According to Aruch ha-Shulchan 96:2, he may not walk to get a siddur even if he knows where one is located. See Hebrew Notes, pg. 269, for discussion).
If one is davening and is in doubt of a halachah concerning the Shemoneh Esrei, he may go and look up the halachah in a sefer. If he has no other choice, he may even ask another person what the halachah is (Mishnah Berurah 104:2 and Kaf ha-Chayim 96:11 quoting the Chayei Adam. Several poskim [R' Shlomo Kluger in Ha-elef Lecha Shelomo O.C. 50; Eimek Berachah, pg. 7] disagree strongly with this ruling even to merely look in a sefer, much less to ask a question. See Yalkut Yosef, pg. 177 who rules like Chayei Adam concerning looking in a sefer. Beis Baruch 25:22 also agrees with the Chayei Adam). This should be relied upon only when not resolving his question might invalidate the Shemoneh Esrei (Beis Baruch 25:22).
1 These sources address a case where the "long version" of a blessing (שים שלום) is converted into a substantially shorter version by design. However, the halachic distinction here seems unclear, since both versions are apparently in the category of ברכה קצרה since this blessing is סמוכה לחברתה and therefore lacking a פתיחה.
2 There seem to be different ways to interpret whether "formulation" and nusach are synonymous here. The Rashba (ibid.) maintains that they are not identical, and that a mere change in the nusach does not invalidate a blessing ex post facto. The Rambam (Hil. B'rachos 1:6) does seem to equate the two, but rules that a change in formulation does not invalidate a blessing unless key elements are missing (i.e. Shem u'malchus). Thus, it seems that the difference between the Rashba and the Rambam might be relegated to semantics. See also this answer.