My two cents is this - SSE is still receiving updates and lacks the script extender needed for more complex and popular mods. This is tentatively due for a beta release next month. However many amazing SSE mods work great right now.
If you plan to install a lot of graphics mods,lighting mods like RLO or ELFX and maybe an ENB preset, the upgraded visuals of SSE become nearly moot as the "new" textures will be replaced anyway. For this reason I recommend sticking with Oldrim.
To anyone new to modding though, Oldrim can be highly unstable and features a number of quirks to be aware of - chiefly a hard cap on VRAM which, if exceeded, WILL crash your game. You cannot go 4K (or even 2k) texture happy and expect things to be fine, even with an 8gb 1080. There are numerous guides to improving stability on the nexus.
That being said, the main advantage of SSE in the long run should be stability and the removal or diminishment of many of those quirks.
Tl:dr - if you want to get modding today, go Oldrim. But by the end of the summer, I expect SSE to nearly have achieved parity between editions.