Hi Wolfgang,
I presume , when you say SMP(its Service Market Place and not SAP Mobile Platform).
Yes you are right, we have 2 ways of installing SAPUI5 plug-in for Eclipse.
If you are going to use plain Eclipse for UI5 project development – Go with Install New Software->.
But if you have NWDS(Netweaver Developer Studio), you can download required plug-in(UI toolkit) from SMP.
Please refer the below link, which will give you insight details about it
regards,
Sabarinathan C