For the original question of the thread title, wether Poland was more Germanic or more Slavic, I would say the answer should be correctly "both": its just the matter of the timing.
Before the Migration Period, the area of modern Poland was predominantly inhabited by Germanic-speaking tribes, such as the Rugians, Burgundians and Vandals. The southwest of modern-day Poland also had a vestigial Celtic element in it, see the tribal name "Lugii", as well as place names such as "Calisia", now Kalisz, and "Lugidunum" (perhaps today Legnica, but that identification is far from certain).
During the Migration Period, the Germanic tribes (in particular the Eastern Germanic ones, including the Goths and the Vandals) entered the territory of the Roman empire and seized - with various successes - parts of it (Vandals in North Africa, Goths in Italy and Spain, Burgundians in Gaul, etc.). In my opinion, its doubtful that the tribes moved as a whole, its more probable that mainly the upper elements of the tribes moved and that a chunk of the population stayed behind and was absorbed by the Slavic tribes in the subsequent centuries (the linguistic evidence, in my opinion, speaks for this). By the 7th century, Slavic tribes inhabited the largest part of former Germanic-speaking Europe, as far west as the modern area of eastern Schleswig-Holstein in the north (bear in mind that the city of Lübeck - one the cities of the Hanseatic league during the Middle Ages, was originally founded by Slavs) to the Elbe and Thuringian Saale rivers in the south. Its clear that before the Eastern Frankish realm (its iffy to genuinely "Germany" at that point already) expanded eastwards, the area of modern eastern Germany (note: I'm talking about the area of the former DDR here, not pre-1945 eastern Germany) was firmly Slavic-speaking. So from that perspective there can be no doubt that the area of modern Poland was predominantly Slavic at the same time. But, wether it was entirely "Polish" or not is a different question, in my opinion.
It must be argued that a distinct Polish identity/ethnicity certainly could not have exist before the Migration Period. And while
speakers of Proto-Slavic no doubt existed (roughly in an area of what today is southeastern Poland, Belarus and the northwestern Ukraine), but we do not know what they called themselves, at least the name "Slav" is unrecorded by Greek or Roman sources
before the Migration Period (yes, Jordanes claims that the Slavs were previously called Venedi, but bear in mind his own work dates from the Migration Period). The word 'Slav' itself is derived from the Slavic word for 'speech' or 'language' (see Russian "слова"/"slova", Polish "słowo"), contrasted with 'mute' or 'non-speaker' (Russian "немой"/"nemoj", Polish "niemy"), which was the exonym used by the Slavic peoples for Germanic speakers (hence, in the modern Slavic languages, the name for "Germany" is derived from this word). If you consider this, it stands to reason that the "Slavic" identity (distinct from the speakers of Proto-Slavic, think of the distinction between "Briton" and "Welsh" here for an analogy) was likely a new construct that emerged in the Migration Period.
Another issue that should be brought up - both with regard for the origins of Poland, and from the perspective of interaction with what was to become Germany, is the issue of religion: the Slavic tribes at the start of the Middle Ages were originally polytheistic (as were the Baltic tribes), while the Franks were Catholics from the 5th century onward. Sadly, you might say that it is no coincidence that the German for 'slave', "
Sklave" is similar to the name "
Σκλαβηνοι" (Sklavēnoi), used first by the Byzantines for the Slavic peoples during the Migration Period. Thus, the Medieval eastwards expansion of East Francia/Germany into Slavic territories was partially driven (it wasn't the only factor, there were also internal factors of early Medieval East Francia that contributed to this) by what you might consider a 'loophole' in the medieval Christian identity, in the way that it was seen as acceptable (even "necessary") to conquer polytheistic peoples (see also Norther Crusades, against the Baltic peoples, in particular the Prussians). This is - somewhat cynically - one of the factors in the Spanish conquests over the native peoples in the Americas. Therefore, what ultimately spared the area that was to become the nucleus of Poland (the Czechs/Bohemia should be brought up here, but that decisively leads offtopic
) from a similar fate as the other Slavic tribes in Central Europe was the adoption of Christianity: the key issue for the origin of Poland is certainly the conversion of Mieshko I. (in 966 AD) to western Christianity / Catholicism, as opposed to the Orthodox church of the Byzantine Empire that adopted by the eastern Slavic peoples (its technically incorrect though to geniunely talk about "Catholicism" before the Church schism).
This is an interesting point, I would like to point out that some of the Sarmatians - the Alans (or
Alanoi, to the Greeks) moved along with the Germanic peoples during the Migration period, the Alans moved together with the Suebi into western Iberia. Purportedly, the origin of the Portuguese town "Alenquer" is "
Church of the Alani".
