I've got my SD2IEC from scitechsuppliesdata (scitechsuppliesltd on ebay), and it's very good, it works like a charm, I can load d64 floppies, but the best is you can have all the data in the sd card (in fat16 format), and load any .prg (like sidwizard) and get the tunes and instruments from the sd card. It's perfect for my use, and the price is fair.

For full compatibility, there's also the the Turbo Chameleon 64 and the UK1541, if you can get your hands on one. They're all quite pricey though. Personally, I'm fine with watching demos off a real floppy drive

Regarding the sd2iec, external power is overkill imo. You'll only ever need that if you want to do diagnose faulty C64s or do hardware projects (in which case you're probably better off with a 1541UII/UK1541/TC64) or use the thing with a non-Commodore machine (in which case there are probably better options available as well).

the sd2iec is adequate for running sid wizard or music players, but if you need full compatibility i think the 1541 ultimate is the only one that'll do it

oh, other thing with the flash/sd carts is to check how much of the disk drive emulation they do. (usually called 1541 emulation after the standard drive)  Some of them only do a kind of 'kernal trap' that won't work with fastloaders.  A lot of music software uses the kernal loader I think, but if you want to run demos/disk games/other things you'd need fast loader support.

Thanks @irrlichtproject, I'll order the second one then (is it the v3?). It seems neither block the port (neither user, cassette or joystick port for the first one), because there is a kind of extender. But having a more recent version is better anyway. And I need an external because I have two C64 with different sound chips.

Any reason to get the v4 instead? They are 50% more expensive (but with a nice case), I'd prefer to spare money to order a gotek for my Atari ST.

Yeah because I have been looking at SD2IEC but Im not sure which version to get. I guess a v4 but then which power supply and extras.

Definately the second one. The first one is an outdated version, and blocking a joystick port is obviously not convenient. I got mine from https://www.thefuturewas8bit.com when they were still selling v3.

About the user port vs cassette port I'm not sure, perhaps it's a matter of leaving the user port available when possible.

Would you recommend this one :
http://www.ebay.fr/itm/SD2IEC-SD-Card-R … Sw3xJXnmGw

or this one :
http://www.ebay.fr/itm/SD2IEC-SD-CARD-C … 5Qd-9mDvXA

(almost the same price, the first one as a case)

for the second one, I don't understand why there are 2 versions, one for the cassette port, one for the user port, they say the cassette port is not compatible with plus-4 or SX-64, but the user port is compatible with everything, so why not only selling the user port version? (maybe an old stock). Is there a benefit to use the cassette port over the user port?

The first version (from Poland) seems to use the joystick port.

I'd use it with SidWizard (and probably other musical tools, but it won't be for much games so I don't really care if some packed games won't work)

SD2IEC.

Ok, great. What is the best SD-card solution or Flashcart righ now?

YES to an SD card interface. IMO it's almost a bare essential for most retro systems to be useful now-a-days. I do respect those that want a 'full experience' with floppy drives  and data recorders and original media. But for cross development, and chip tuning with PC tools like GoatTracker, you need some (fast) way to move between the systems. Here is some basic info-
http://www.c64-wiki.com/index.php/SD2IEC
You could also go the Flash Cart route.
Yogi

c64 I think better for music making, c64c and c128 are much bigger)

c64 I think better for music making, c64c and c128 are much bigger)

Awesome guys great feedback. Ive gotten hold of a newer C64 with a mssiah cart so Ill start there.
What do I need extra if I want to use any other tracker?`Or any other  tips. Like is it a good idea to have a SD card reader?

Mssiah is clunky as f*ck indeed - but in a live show setting the interface does have some pretty nice quirks that trackers don't really allow easily.

Personally I don't use Mssiah / Prophet for midi chains anymore (bassline / monosynth modules) - but the sequencer mode with 2xSIDs makes for a pretty nice environment for a live setup where you have reasonably high level of control over filters / SID quirks, and with the magic of Scannerboy's DIN / midi sync I can hang my electribes and other midi devices using Prophet64 as the master clock which is nice!